Can A Firefox Extension Keep AI Chat Data After You Clear A Private Session?
Yes. A Firefox extension allowed in Private Windows can copy AI chat data into extension storage that survives Clear Private Session. The fire button clears private website data, not every extension-controlled copy.
title: "Can A Firefox Extension Keep AI Chat Data After You Clear A Private Session?" slug: "firefox-extension-private-session-ai-chat-data" excerpt: "Yes. A Firefox extension allowed in Private Windows can copy AI chat data into extension storage that survives Clear Private Session. The fire button clears private website data, not every extension-controlled copy." meta_title: "Can Firefox Extensions Keep Private AI Chat Data?" meta_description: "Learn why Firefox Clear Private Session may not delete AI chat data copied by an extension, which storage survives, and how to test the real deletion boundary." tags: - Firefox extensions - private browsing - AI chat privacy - extension storage - browser-local history status: published published_url: "https://openveil.app/blog/firefox-extension-private-session-ai-chat-data" published_at: "2026-08-04" post_id: "33be2a58-6082-407c-bdbd-898810437858"
Watch The 30-Second Summary
Yes. A Firefox extension that is allowed to run in Private Windows can copy AI chat content into extension-controlled storage that survives Firefox's Clear Private Session button. Firefox 151's fire-button reset clears the current private-browsing session, including temporary website cookies and site data. It is not a universal erase command for data an extension saved through storage.local, synchronized through storage.sync, downloaded to disk, sent to a remote service, or passed to a native application.
That does not mean every extension keeps your chats. It means the answer depends on four facts: whether the extension could run in Private Windows, what page access it had, what it actually collected, and where it wrote the copy.
The Short Answer: Separate The Page From The Extension
Firefox Private Browsing and Firefox extensions do not share one deletion boundary.
| Possible copy | Can an extension create it? | Does Clear Private Session reliably remove it? |
|---|---|---|
| AI app's cookies or IndexedDB inside the current Private Browsing session | It may be able to read or influence some page data, depending on permissions | Usually yes for the private-session website copy |
Extension storage.local data |
Yes | No; MDN says this API is designed to persist through browser history and data clearing |
Extension storage.sync data |
Yes | No; it can be synchronized through the browser to other signed-in desktop instances |
Extension memory only, using storage.session or ordinary runtime state |
Yes | It depends on the extension lifecycle, not the website's private-session reset alone |
| File downloaded or exported to the computer | Yes, with the needed capability and user flow | No |
| Data sent to an extension developer, analytics service, AI API, or other server | Yes, if the extension has the necessary network access and does so | No |
| Data passed to a native helper application | Yes, with native-messaging permission and an installed helper | No |
The practical rule is simple:
Clear Private Session controls Firefox's temporary private-browsing data. It does not prove that every program which observed the session deleted its own copies.
Why An Extension Can See A Private AI Chat
Firefox does not enable ordinary extensions in Private Browsing by default. Mozilla's current Extensions in Private Browsing guide says the user must grant an extension permission to run in Private Windows, either during installation or later in the Add-ons Manager.
That permission is a meaningful safety boundary. If an extension is set to Don't Allow under Run in Private Windows, Firefox says it should not operate in those windows. If it is allowed, it may be able to act there according to its other permissions.
Private-window access alone does not reveal chat text. An extension also needs a way to observe the page or the user's actions. Depending on its design and granted permissions, that could include:
- a content script on the AI site's origin
- permission to read or modify data on matching websites
- a sidebar, popup, or context-menu action that the user invokes
- clipboard access after text is copied
- a page-injected assistant, grammar checker, summarizer, translator, or note-taking tool
- broad access to all websites rather than one named AI domain
Mozilla warns in the same guide that an extension running in private windows can be problematic when it has access to information the user wants to keep private. The warning is not an accusation against every add-on. It is recognition that Private Browsing cannot make an authorized observer forget what it saw.
Why storage.local Can Survive The Fire Button
The name storage.local sounds as though it belongs to the current webpage. It does not. It is a WebExtension storage area owned by the extension.
MDN's storage.local documentation says its items are local to the machine where the extension is installed. More importantly, MDN distinguishes this API from Window.localStorage: Firefox can clear extension localStorage in privacy-related browsing-data scenarios, while data written through the WebExtension storage.local API is designed to persist in those scenarios.
That distinction creates two separate containers:
- Website storage: cookies, cache, IndexedDB, and related data belonging to the AI website's origin in the private session.
- Extension storage: records belonging to the add-on's identity, managed through the WebExtension storage API.
Firefox's Clear Private Session button resets the first container. It does not automatically call browser.storage.local.clear() for every installed extension.
An extension developer can choose to delete its own saved data when a private session ends or when the user presses an extension-specific erase control. But that behavior must be implemented and verified. The browser fire button is not evidence that the extension did it.
Firefox Uses A Spanning Extension Model
The storage boundary is easier to understand after looking at Firefox's extension execution model.
MDN's incognito manifest documentation describes the default spanning model: one extension instance can receive events from ordinary and private windows, with each tab or window marked as private or non-private. The extension can use that signal to avoid collection, separate records, or label them correctly.
Firefox does not support the alternative split mode that some Chromium-based browsers use. In split mode, private and non-private extension execution can be isolated more strongly, although MDN still notes that WebExtension storage.local is shared. Firefox installs an extension requesting split mode as not allowed in private windows.
For a Firefox user, the important consequence is this:
An allowed extension can operate across both ordinary and private contexts, and its durable extension storage is not automatically partitioned into a temporary private-session vault.
Whether private content is retained therefore depends on the add-on's code and policy, not merely the purple-window indicator.
Could The Extension Sync The Copy To Another Device?
Yes, if the extension deliberately uses storage.sync and the Firefox account configuration permits add-on synchronization.
MDN's storage.sync documentation says data in that area is synchronized by the browser and becomes available to other signed-in desktop instances. Firefox users must have Add-ons selected in Sync. The API has quotas and is mainly intended for preferences, but those limits do not prevent an extension from storing short notes, titles, identifiers, snippets, flags, or compact conversation metadata.
Mozilla's Firefox Sync settings guide separately says users can synchronize add-ons across desktop devices. That does not mean every add-on's data is synced, or that Firefox Sync uploads complete AI transcripts by default. It means an extension can have a browser-supported cross-device storage path when its developer uses it.
If a sensitive snippet was synchronized before the private session was cleared, resetting the source window would not recall every synchronized copy. The extension would need to remove the key from sync storage, and the deletion would need to propagate to the other instances.
What Clear Private Session Still Does Well
This extension caveat does not make Firefox's control useless.
Mozilla's Private Browsing guide says Firefox 151 can wipe the current private-browsing data and open a fresh private session without closing the entire browser. The same guide explains that cookies, visited pages, form entries, download-list entries, cache, and offline website data are not retained in the ordinary way after a private session ends.
For an AI app whose conversation history exists only in private-session website storage, the fire button is a strong and convenient reset. It can remove the local website copy and temporary sign-in state. It also reduces the chance that someone using the same Firefox profile will reopen the private AI page and recover the session from that temporary store.
The limitation is scope. Firefox explicitly notes that bookmarks and downloaded files survive Private Browsing. Extensions are another separate scope. So are provider databases, network logs, crash reports, clipboard managers, screenshots, operating-system backups, and anything pasted into another app.
A Seven-Step Extension Audit
Use harmless test content. Do not paste real sensitive information merely to see where it leaks.
1. List The Extensions Allowed In Private Windows
Open Firefox's Add-ons Manager, select each extension, and inspect Run in Private Windows.
Pay special attention to:
- writing assistants and grammar tools
- AI sidebars and browser copilots
- note-taking, clipping, translation, and summarization extensions
- password, form-filling, and clipboard tools
- automation, accessibility, and developer extensions
- any extension with broad access to all websites
An extension that is not allowed in Private Windows is outside this particular path. It may still hold earlier data collected in ordinary windows.
2. Review Website And Optional Permissions
Private-window access answers where the extension may run. Host and feature permissions answer what it may do.
Check whether the add-on can read and change data on the AI site's domain or on all websites. Review optional permissions that may have been granted after installation. A narrow toolbar tool that runs only when clicked is a different risk from an always-on content script with broad site access.
Permission review establishes capability, not actual collection. Continue to the storage and network tests.
3. Read The Add-On's Privacy Notice And Data Controls
Look for concrete answers:
- Does it process text locally or send it to a service?
- Does it retain snippets, diagnostics, prompts, or page URLs?
- Does it use browser sync?
- Can private-window collection be disabled separately?
- Is there an erase or account-deletion control?
- What happens to backups and derived data?
Vague statements such as “we respect privacy” do not define a deletion boundary.
4. Create A Unique Harmless Canary
In a Firefox Private Window, enter a distinctive phrase into a non-sensitive test chat, such as:
firefox-extension-canary-2026-08-04
Use the AI app and extension normally. If the extension has an explicit save, summarize, clip, or rewrite action, test the feature you actually use.
5. Clear The Private Session
Use Firefox's fire button and confirm the reset. Reopen the AI app in the fresh private session.
If the conversation disappeared from the AI app, the website-storage reset worked. That result says nothing yet about extension storage.
6. Search The Extension's Own Surfaces
Open the extension's popup, sidebar, history, dashboard, saved-items view, settings, and any signed-in web account. Search for the canary.
If it remains, document where it appears and use the extension's own deletion control. If it reappears on another desktop Firefox instance, investigate browser sync or the extension's server account.
Developer Tools can provide deeper evidence for a controlled test, but avoid changing internal extension data unless you understand the impact. Ordinary users can often establish the boundary from the add-on's visible history and account dashboard.
7. Remove Every Copy And Retest
Delete the canary from the extension, its sync or web account, the AI app, downloaded files, and other destinations. Then repeat the test with the extension disabled in Private Windows.
A clean retest should show two things:
- the AI app's temporary private-session copy disappears after the Firefox reset
- no extension-controlled copy appears because the add-on could not observe that session
That is stronger evidence than assuming one browser button governed both systems.
What Extension Developers Should Do
An extension that supports Private Browsing should make its behavior explicit rather than treating private and ordinary content identically.
Useful design controls include:
- defaulting to no retention for data observed in private windows
- checking the private/incognito flag before collecting or saving content
- keeping private-session state in memory when durable storage is unnecessary
- separating private-derived records and giving them a visible expiration rule
- avoiding
storage.syncfor sensitive page content - providing a one-click erase control that covers local, synchronized, and server-side copies
- documenting whether telemetry, crash reports, or third-party AI providers receive page content
- minimizing host permissions and requiring a user action for sensitive-page access
The word “local” also needs precision. Extension storage.local means local to the installed browser profile, not temporary, private-session-only, encrypted, inaccessible to other local software, or automatically deleted by the fire button.
Where OpenVeil Fits
OpenVeil keeps normal private-chat history in the browser and does not maintain a server-side chat-history record for those sessions. That gives users a clearer history-storage boundary than products built around a persistent cloud transcript.
An extension can still create a separate copy. OpenVeil cannot force a Firefox add-on, clipboard manager, screenshot tool, malware process, or other observer to forget content it was allowed to access. For sensitive work, use a clean browser profile, minimize extensions, and deny Private Window access to add-ons that are not necessary.
OpenVeil is also hosted, not fully offline or anonymous. Active requests can still be processed by OpenVeil and necessary AI, search, upload, hosting, routing, security, billing, and infrastructure providers. Clearing browser-local history does not reverse processing that already occurred.
If that documented boundary fits your task, review OpenVeil's privacy policy and compare it with the extension permissions in the browser profile you plan to use.
FAQ
Does Firefox Clear Private Session disable extensions?
No. It resets the current private-browsing data and opens a fresh private session. An extension that remains allowed in Private Windows can run again in the new session.
Does clearing Firefox history delete storage.local for extensions?
Do not assume so. MDN specifically recommends the WebExtension storage.local API because its data persists in browser history and data-clearing scenarios where extension Window.localStorage may be removed.
Are Firefox extensions enabled in Private Browsing by default?
Ordinary extensions are not enabled there by default. Mozilla says the user must grant permission. Some built-in or specially controlled components may follow different rules.
Can an extension read every AI chat just because it runs privately?
No. It needs relevant page, host, feature, or user-granted access, and its code must actually collect the data. Permission indicates possible authority, not proof of behavior.
Does uninstalling the extension delete its local extension storage?
MDN says uninstalling an extension clears its associated local storage under normal settings. That does not delete remote service records, synchronized copies already present elsewhere, downloaded files, or data exported to another application.
Is a private window safe for sensitive AI work?
It can reduce local browser traces, but it does not make the user anonymous or block authorized extensions and other software. Use a trusted browser profile with minimal extensions, understand the AI provider's data path, and avoid entering information whose disclosure would be unacceptable.
Bottom Line
Firefox's Clear Private Session button can remove an AI app's temporary private-session website data. It does not prove that an allowed extension deleted what it copied. Extension storage.local, storage.sync, downloads, native applications, and remote services each have their own retention and deletion controls.
Before using a Private Window for sensitive AI work, review which extensions can run there. After clearing a test session, check the extension's own history and account surfaces with a harmless canary. Privacy is determined by every copy and every observer—not by the label on one window.