Can A Firefox Extension Sync Private AI Chat Data To Another Device?
Yes, if an allowed Firefox extension writes chat data to sync storage or its own cloud. Private windows do not make extension copies session-only.
Yes—a Firefox extension can potentially make AI chat data available on another signed-in desktop Firefox instance, including data the extension handled while it was allowed to run in a Private Window. But Firefox does not automatically sync every private chat, and syncing an add-on is not the same thing as syncing its data.
The result depends on what the extension can read, which storage API it uses, whether Firefox Sync is enabled with Add-ons selected, and whether the extension sends anything to its own service. Closing the Private Window only clears Firefox's private-session browsing data. It does not prove that an extension-controlled local copy, synced copy, export, log, or provider-side copy disappeared.
What Is Confirmed
Mozilla's current documentation establishes five important facts.
First, extensions are not allowed to run in Firefox Private Windows by default. Mozilla's Extensions in Private Browsing guide says the user must grant that access during installation or later in the extension's settings. Once allowed, an extension may be able to access information from private pages within the permissions it has been granted.
Second, Firefox WebExtensions have storage that is separate from a website's own cookies and site storage. MDN's WebExtensions storage documentation describes extension-specific local, session, managed, and sync storage areas.
Third, storage.local persists on the machine. MDN says data written through the WebExtensions storage.local API is local to the machine and can persist in situations where Firefox clears extension localStorage while clearing browsing data. In other words, clearing private browsing data is not a general extension-data deletion command.
Fourth, storage.sync can travel. MDN's storage.sync reference says items in that area become available on browser instances where the user is logged in. For desktop Firefox, the user must have Add-ons selected in Firefox Sync. MDN currently says Firefox for Android does not synchronize this extension data.
Fifth, Firefox does not isolate WebExtensions storage.local between private and non-private operation. MDN's incognito manifest documentation says Firefox does not support split mode and that WebExtensions storage.local is shared. An extension that saves a private-window conversation to that store can therefore create a durable record outside the private tab's ordinary cookies and history.
These facts establish capability, not behavior by every extension. An extension must actually read the chat and choose to save it before there is chat content to sync.
What Is Still Unclear
Firefox's settings cannot tell you, by themselves, what a particular AI extension stores. The following questions are extension-specific:
- Does it read the prompt, response, page text, attachments, or only a harmless preference?
- Does it use
storage.session,storage.local,storage.sync, IndexedDB, downloads, native messaging, or its own server? - Does its
storage.syncpayload contain chat content, an identifier, a summary, or only settings? - Does it encrypt stored data before writing it or transmitting it?
- Does a deletion command remove all extension-controlled copies and propagate to other devices?
- Does the extension have an account-level cloud history independent of Firefox Sync?
- Does uninstalling it remove remote or already-synchronized copies?
- Can browser-profile backups, device backups, telemetry, crash reports, or exports retain another copy?
Mozilla describes storage.sync as primarily useful for extension preferences. That is a design recommendation, not a technical guarantee that every developer stores only preferences. MDN also warns that the extension storage area is not encrypted and should not be used for confidential information.
The safe conclusion is therefore conditional: Firefox provides a path by which extension data can sync, but only the extension's code, documentation, and a controlled test can show whether private AI chat content uses that path.
Three Different Things People Call “Extension Sync”
The phrase “my extension synced” can describe three different events.
| Event | What it can do | What it does not prove |
|---|---|---|
| Add-on installation sync | Installs or maintains compatible desktop extensions across signed-in Firefox instances | That the extension's chat data moved |
| Extension data sync | Makes values written to browser.storage.sync available on other signed-in desktop instances |
That every extension value or private chat was included |
| Extension account sync | Uses the developer's own service to move settings, chats, or files | That Firefox Sync controlled, encrypted, retained, or deleted the data |
Mozilla's Firefox Sync guide lists Add-ons as a desktop sync category and explains that Sync keeps a combined copy on a remote server and synchronizes it with connected devices. The separate MDN storage.sync documentation explains the extension-data channel.
An extension can also build its own account system. That path may work even when Firefox Sync is off. Conversely, an extension can appear on a second desktop because Add-ons sync installed it while none of its chat content follows.
Do not use the presence or absence of the extension icon as proof about conversation data.
Why Private Browsing Does Not Set The Extension's Retention Policy
Firefox Private Browsing limits what Firefox retains from the browsing session. Mozilla's Private Browsing guide says Firefox does not save browsing information such as history and cookies after the private session ends. It also emphasizes that Private Browsing does not make a user anonymous and does not stop websites, employers, internet providers, or local spyware from observing activity.
An allowed extension is another participant in that session. It can operate under its granted permissions, and its storage is not the same store as the private page's cookies.
That creates a simple but important boundary:
A Private Window controls Firefox's treatment of the private browsing session. It does not automatically impose a no-save rule on every extension allowed into that window.
For example, imagine a sidebar extension that captures an AI response so the user can organize it later. It could:
- keep the text only in memory using
storage.session; - save it on the current machine using
storage.local; - save selected fields through
storage.sync; - download a file;
- send it to the extension developer's service; or
- combine several of those methods.
Closing the Private Window has a different effect in each design. Only the first approach is inherently session-scoped by the named storage mechanism. The others require their own retention and deletion rules.
The CROSS Audit For A Firefox AI Extension
Use this five-step audit before trusting an extension with a sensitive private chat.
C — Check Private-Window Access
Open Firefox's Add-ons Manager, select the extension, and find Run in Private Windows. If it is set to Don't Allow, the extension should not receive access to private tabs through ordinary extension operation. If it is set to Allow, treat it as part of the private-session trust boundary.
Permission is not proof of collection. It is proof that the extension may operate there.
Also review the extension's site permissions. Mozilla's extension permission guide explains that broad website access can allow an extension to read page content and information entered into pages. Check whether access is limited to the AI site you intend to use or applies to every site.
R — Review Storage And Transmission Paths
Read the extension's privacy notice, support documentation, source repository, and manifest when available. Search for specific statements about:
- prompts and responses
- website content
- files and attachments
storage.local,storage.sync, and IndexedDB- analytics, diagnostics, and crash reporting
- remote accounts and cloud backup
- retention and deletion
- model training
- subprocesses or native messaging
A generic claim such as “your data stays private” is not enough. Look for which data, where it is stored, when it leaves the browser, and how every copy can be deleted.
Developers and advanced users can inspect extension storage from Firefox's developer tools through about:debugging, as MDN documents. That can identify visible keys and values, but it does not prove what was transmitted over the network or retained by a remote service.
O — Observe A Harmless Local Copy
Create a disposable conversation with a unique canary phrase, such as CROSS-TEST-81426, containing no real name, secret, client detail, or other sensitive data.
With Firefox Sync temporarily out of the test, use the extension in a Private Window, close every Private Window, then reopen Firefox normally. Check whether the extension still displays, searches, exports, or otherwise recalls the canary.
If it does, private-session closure did not remove the extension's copy. That is useful evidence about the tested version and configuration, not a universal conclusion about the product.
S — Sync The Canary To A Second Desktop Profile
If you intend to use Firefox Sync, test with two desktop Firefox instances signed into the same Mozilla account. Confirm that Add-ons is selected under Sync, install or sync the extension, and use Sync Now if appropriate.
Mozilla says Firefox extension data sync normally occurs every ten minutes or when the user selects Sync Now. Check whether the complete canary chat, a title, a preference, or nothing appears on the second desktop.
Repeat only with disposable data. A successful test creates at least one additional copy, and deleting the source immediately can make it harder to understand which device or sync event changed the result.
S — Scrub Every Known Copy
Delete the canary through the extension's documented controls. Then verify separately:
- the original Firefox profile
- the second desktop profile
- the extension's remote account, if any
- Firefox Sync settings and connected devices
- downloaded files or exports
- browser and device backups
- the underlying AI service's own chat history
Mozilla says turning off all Firefox Sync categories deletes previously synced data from Mozilla's servers but does not delete local data already present on devices. That is why “turn Sync off” and “erase every copy” are not equivalent instructions.
If the extension offers no documented full-delete path, do not use a real secret to finish the experiment.
A Practical Risk Matrix
| Extension behavior | Cross-device risk | Private-window takeaway |
|---|---|---|
Uses only storage.session for the conversation |
Lower after the extension session ends | Still verify remote requests and logs |
Writes conversation text to storage.local |
Persists on the current machine | Closing the Private Window may leave the extension copy |
Writes conversation text to storage.sync |
Can become available on other signed-in desktop Firefox instances | Private browsing does not make the synced value session-only |
| Uses its own cloud account | Depends on the developer's service, not merely Firefox Sync | Review the developer's retention, training, and deletion rules |
| Downloads or exports chats | Creates a file outside browser storage | Protect backups, search indexes, and recycle-bin copies |
| Stores only a UI preference | Usually low chat-content exposure through that value | Do not infer the same behavior for future versions or other features |
This matrix is about storage paths, not whether an extension is trustworthy overall. A well-designed extension can use sync responsibly. A poorly designed extension can leak data without using sync at all.
What Administrators Should Check
Organizations should not treat Private Browsing as a substitute for extension governance. For managed devices:
- maintain an approved-extension list;
- review host permissions and private-window access;
- separate extensions that process confidential text from general browsing tools;
- record whether Add-ons sync is allowed under organization accounts;
- inspect vendor data-flow, retention, deletion, and subprocessors;
- test changes after extension updates;
- restrict native messaging and broad website access where unnecessary;
- define whether exports and browser-profile backups are allowed;
- provide a non-sensitive canary procedure for verification; and
- treat a synced extension copy as organizational data, not personal browser clutter.
An extension update can change permissions, storage design, or service behavior. Re-run the audit after a material version change rather than relying forever on one clean test.
Where OpenVeil Fits
OpenVeil is a hosted, privacy-focused AI workspace for chat, files, search, voice, images, and video. Its documented normal private-chat history is browser-local, and OpenVeil does not create a server-side normal chat-history record for those sessions. OpenVeil also documents that prompts, uploads, media, and outputs are not used to train foundation models.
That boundary does not make OpenVeil fully offline or prevent an extension from reading a page the user has allowed it to access. Active requests still require processing by OpenVeil and necessary providers, and operational account, billing, security, routing, and infrastructure records still exist.
If you use OpenVeil in Firefox, an allowed third-party extension can create a separate data path outside OpenVeil's browser-local history design. The safest pattern for a sensitive session is to disable unnecessary extensions in Private Windows, use a dedicated browser profile with a minimal extension set, and export only the work you intentionally want to retain.
Read how Firefox extensions can retain data from a private AI session, learn why browser sync does not automatically back up browser-local AI history, and use the checklist for evaluating AI privacy claims before sharing sensitive information.
Frequently Asked Questions
Does Firefox Sync Every Extension's Private Chat Data?
No. Firefox can sync values that an extension writes to storage.sync, but an extension may use a different storage method or store no chat content at all. The extension's implementation determines the payload.
Does Selecting Add-ons In Firefox Sync Only Install Extensions?
No. It can support both compatible add-on synchronization and the storage.sync data area used by extensions. That does not mean every installed extension syncs its content.
Can Firefox Extension Data Sync To Android?
MDN currently says Firefox for Android does not synchronize WebExtensions storage.sync data with the user's account. Desktop extension compatibility and installation are also distinct from mobile support. Check current Mozilla documentation for the devices you use.
Does Closing Every Private Window Delete Extension Storage?
Not necessarily. Private cookies and ordinary private-session browsing information are cleared, but WebExtensions storage.local can persist and storage.sync can create a synchronized copy. The extension may also use another local or remote store.
Can An Extension Read An AI Chat In A Private Window?
Potentially, if the user allowed the extension to run in Private Windows and its granted permissions cover the page or relevant browser APIs. Permission establishes access potential, not proof that the extension collected the chat.
Is Firefox storage.sync Encrypted?
Mozilla documents encryption for Firefox Sync transport and server-side data handling, but MDN warns that the WebExtensions storage area itself is not encrypted and should not contain confidential information. Do not treat the API name as a guarantee of application-level end-to-end encryption for a chat payload.
Will Turning Off Firefox Sync Delete The Copy On Every Device?
No. Mozilla says disabling all synced categories removes previously synced data from Mozilla's servers, but local data already present on connected devices remains. Review each device and the extension's own account separately.
Does Uninstalling The Extension Delete Its Remote Account Data?
Do not assume so. Uninstalling may remove local extension data, but it does not prove that the developer's servers, Firefox Sync, another device, an export, or a backup deleted their copies. Follow the documented account-deletion and data-deletion process.
Can OpenVeil Stop A Firefox Extension From Copying A Chat?
OpenVeil's browser-local history and no-foundation-model-training boundaries do not control a third-party extension that the user allowed to access the page. Browser extension governance remains the user's or administrator's responsibility.
The Bottom Line
A Firefox Private Window is not a sealed container once an extension is allowed inside it. Firefox can clear private-session cookies and history while extension-controlled data remains in storage.local, moves through storage.sync, appears in an export, or reaches the extension developer's own service.
Do not assume that extension installation sync means chat sync, and do not assume that closing a Private Window means extension deletion. Check private-window access, review the extension's actual storage and transmission paths, test a harmless canary across two desktop profiles, and verify deletion on every known copy before trusting the workflow with sensitive AI chat data.