Can Firefox Sync Overwrite AI Extension Data From Another Device?

August 22, 2026

Firefox Sync can overwrite a newer local AI-extension edit. Learn when server data wins, what encryption cannot restore, and how to test safely.

Can Firefox Sync Overwrite AI Extension Data From Another Device?

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Yes. Firefox can overwrite a local change made by an AI extension when that extension stores the same key in storage.sync on multiple desktop browsers. Mozilla documents that a server-side change takes precedence for each key during synchronization. Whether this can affect your chats, drafts, personas, indexes, or only preferences depends on how the specific extension stores and merges its data.

Who This Is For

This guide is for people who use a Firefox extension to summarize pages, save prompts, keep AI drafts, maintain a local knowledge index, or add an assistant beside normal webpages.

It is especially relevant when:

the same extension is installed on two desktop computers;
both browsers use the same Mozilla account;
Add-ons is selected in Firefox Sync;
the extension says that settings, notes, or other data follow you across devices; or
you have treated synchronized extension data as a backup.

The important distinction is not whether Firefox Sync is private. The immediate question is whether two devices can safely edit the same extension record before both have synchronized.

What Is Confirmed

Mozilla's current storage.sync documentation confirms five details that matter here:

Data placed in an extension's storage.sync area can become available to other signed-in desktop browser instances.
Desktop Firefox synchronizes extension data about every 10 minutes or when the user selects Sync Now.
Firefox compares records one key at a time.
When a server value changed since the browser's last sync, Firefox writes that server value into the browser's sync storage.
A server-side change therefore takes precedence over a local change to the same key.

MDN gives the resulting conflict explicitly: one browser can update and synchronize a key, while a second browser changes its local copy before synchronizing. When the second browser next syncs, its local update can be overwritten.

Mozilla also documents tight quotas for this storage area: 102,400 bytes total, 8,192 bytes per item, and 512 items per extension. Those limits reinforce the intended use. storage.sync is best suited to compact preferences and other globally shared state, not a large archive of full AI conversations.

A Two-Device Overwrite Timeline

Assume an extension keeps all of its draft state under one key named workspace.

Step	Device A	Device B	Sync server
1	Has revision 1	Has revision 1	Has revision 1
2	Changes the key to revision 2A and syncs	Still has revision 1	Now has revision 2A
3	No new change	Changes its unsynced copy to revision 2B	Still has revision 2A
4	No new change	Runs Sync Now	Revision 2A wins for that key

The second device's 2B edit can disappear even though it was the most recent human action. From Firefox's perspective, the server changed after Device B's last synchronization, so the server record is authoritative for that conflict.

This does not mean every simultaneous edit is lost. If the extension stores independent records under different keys, Device A can change one key while Device B changes another without creating the same-key conflict. If the extension places an entire AI workspace inside one large object, however, a change to one draft can collide with an unrelated change elsewhere in that object. That key-granularity conclusion follows from Mozilla's documented per-key synchronization process; the actual result depends on the extension's schema.

Encryption Does Not Turn Sync Into A Versioned Backup

Mozilla says Firefox Sync uses end-to-end encryption: data is encrypted before leaving a browser, and Mozilla cannot decrypt the synchronized contents on its server.

That protects confidentiality against the sync service. It does not create:

a user-visible revision history for every extension key;
automatic three-way merging of two changed JSON objects;
a recycle bin for an overwritten draft;
protection from an extension writing the wrong value; or
proof that a deleted or replaced value can be restored later.

Encryption answers who can read the synchronized copy. Conflict handling answers which copy survives. Backup answers whether an older copy can be recovered. Those are three separate properties.

Mozilla's general extension-storage guidance also says the browser storage area itself should not be used for confidential information without additional protection. Firefox Sync encrypts synchronized records in transit to and at the sync service, but an extension still has to consider readable data in the local browser profile, its own permissions, exports, logs, and any separate servers it uses.

Add-On Sync And Extension-Data Sync Are Related But Not Identical

Firefox exposes Add-ons as a Sync category. Mozilla's current Sync settings guide says selecting it synchronizes desktop extensions across devices. MDN adds a narrower implementation detail: Firefox's storage.sync relies on the extension's Add-on ID.

That creates several distinct layers:

Extension installation: whether the add-on itself appears on another desktop.
Extension sync storage: the compact keys the add-on deliberately writes to storage.sync.
Extension local storage: device-local data written to storage.local.
Website storage: localStorage, IndexedDB, cookies, or other data owned by the AI website rather than the extension.
Browser history and bookmarks: separate Firefox data types that may contain AI-page titles or URLs.
The extension's own account service: a developer-operated cloud sync path, if the product has one.

An installed extension does not prove that all of its data is synchronized. A synchronized preference does not prove that full chat content is synchronized. Conversely, an extension can use its own account server even when Firefox's Add-ons category is disabled.

Firefox for Android adds another boundary. MDN says Firefox's extension storage.sync data does not synchronize with the user's account on Android. Mozilla separately says desktop WebExtensions and settings are synchronized only among desktop Firefox instances. A desktop conflict test therefore should not assume that an Android installation participates in the same data path.

Why AI Extension Data Makes The Conflict More Important

Losing a color preference is annoying. Losing or reverting AI state can be materially different.

An extension might place any of these compact representations in one synchronized key:

a prompt template;
persona instructions;
a list of conversation titles or identifiers;
a note or unfinished draft;
an index of locally stored chats;
a list of connected websites;
a model, search, or privacy preference; or
a pointer to content stored somewhere else.

The sync quota makes full transcripts less likely, but it does not make the metadata harmless. A title, client label, research topic, or persona instruction can be sensitive even when the complete conversation lives elsewhere.

An overwritten index can also make content look deleted when the underlying local records still exist. The reverse is possible too: an older synchronized index can make a removed item appear to return even though no remote service restored the original message body. The user-visible symptom alone does not reveal which storage layer changed.

What Is Still Unclear

Mozilla documents Firefox's synchronization behavior. It cannot answer how an unnamed AI extension is built.

Before concluding that Sync overwrote your data, you still need to determine:

whether the extension uses storage.sync at all;
which top-level keys it writes there;
whether it stores full content, metadata, settings, or references;
whether it listens for storage.onChanged and merges remote changes;
whether records include revision numbers, timestamps, device identifiers, or tombstones;
whether the extension keeps local snapshots or exports;
whether a separate developer cloud account is also involved; and
whether the apparent loss came from Sync, the extension's own code, an update, an uninstall, a profile change, or ordinary website storage.

Firefox also does not promise that selecting Sync Now creates a durable restore point. Mozilla describes Sync as a combined server-and-device state that keeps connected browsers up to date. That is not the same contract as a historical backup service.

Run A Harmless CONFLICT Test

Do not test with a real client note, personal journal, medical detail, password, API key, or irreplaceable AI draft. Use an unmistakably synthetic value.

The CONFLICT test makes the data path observable:

C — Create a synthetic record. In the extension, save a harmless value such as SYNC-TEST-BASE-2026-08-21.
O — Observe both desktops. Run Sync Now on Device A and Device B. Confirm the same baseline appears on both.
N — Network-isolate Device B. Turn off its network connection or close Firefox before the next edit so it cannot silently synchronize.
F — Fork the value. Change Device A to SYNC-TEST-A, reconnect it, and run Sync Now. While Device B is still isolated, change its local copy to SYNC-TEST-B.
L — Log the evidence. Record the device, local time, extension version, Firefox version, and visible value. Screenshots are useful, but do not capture real sensitive content.
I — Initiate Sync on Device B. Reconnect it and run Sync Now once. Do not keep editing while the result settles.
C — Compare the surviving value. Check whether B kept its local edit, accepted A's server copy, merged both, duplicated the record, or showed an error.
T — Test recovery separately. Look for an extension export, snapshot, audit log, or documented restore path. Do not assume Firefox can recover the displaced value.

This test establishes what the extension actually does with one record under your current versions and settings. It does not prove that every record follows the same key layout, that a future extension version will behave identically, or that all devices completed synchronization at the times shown.

What Extension Developers Should Do

Mozilla recommends listening for sync changes when data must be aggregated across devices. For an AI extension, a robust design usually needs more than accepting whichever JSON object is received last.

Developers should consider:

keeping large chats and confidential content out of storage.sync;
using separate keys so unrelated edits do not collide inside one giant object;
adding immutable record IDs rather than using a title as the identity;
recording revisions and device-independent update times;
handling storage.onChanged events explicitly;
preserving tombstones long enough to distinguish deletion from an old-device replay;
presenting a conflict instead of silently choosing a copy when both changed;
offering export and restore independently of browser sync; and
testing offline edits, delayed devices, reinstalls, upgrades, quota failures, and simultaneous Sync Now actions.

Calls to storage.sync.set() are asynchronous and can be rejected when quotas are exceeded. A successful save in the extension UI is therefore not sufficient proof that the value reached Firefox's sync storage or another device. Error handling and visible sync status matter.

What This Means For OpenVeil

OpenVeil is a hosted, privacy-focused AI workspace for adults. Normal OpenVeil chat history is kept in the user's browser, and OpenVeil does not maintain a normal server-side chat-history record for those private sessions. OpenVeil is not a Firefox extension sync service, and browser-local history should not be described as a cross-device backup.

Active prompts, selected local history context, uploads, search, voice, images, video, and outputs still require processing by OpenVeil and necessary providers. OpenVeil also keeps the account, billing, security, and operational records needed to run the service.

The practical connection is narrower: if you want a hosted AI workspace without operating a local model server or depending on an extension's synchronized chat archive, OpenVeil offers browser-local normal chat history plus chat, search, files, voice, and media tools. Clearing browser data, changing profiles, or moving to another device can still hide or remove that local history, so export anything you cannot afford to lose.

You can compare this boundary with whether browser sync backs up browser-local AI history, how Firefox extensions can sync private AI data, and how extension storage can survive a cleared private session.

Frequently Asked Questions

Can Firefox Sync overwrite an AI draft on another computer?

It can if the extension stores that draft—or an object containing it—under the same storage.sync key on both desktops and does not implement its own conflict handling. Mozilla says a changed server value takes precedence over a changed local value for the same key.

Does the newest human edit always win?

No. Firefox's documented process is not simply “latest wall-clock timestamp wins.” A server change detected since a browser's last sync is written into that browser first, so a newer unsynchronized local edit can lose.

Does Firefox merge two changed objects automatically?

Mozilla describes conflict handling per key, not a field-by-field merge inside an arbitrary extension object. An extension can listen for changes and implement its own merge logic, but you must inspect or test that extension to know whether it does.

Is Firefox Sync a backup for AI extension data?

Treat it as synchronization unless the extension separately documents versioned backup and restore. End-to-end encryption protects synchronized content from the service provider; it does not guarantee old-version recovery.

Does disabling Add-ons Sync delete local extension data?

Not necessarily. Mozilla says disabling a Sync category removes that category's data from Mozilla's servers, but local extension data and an extension developer's separate cloud records are different stores. Verify each layer before deleting anything.

Can Firefox Android overwrite the desktop extension's sync data?

MDN currently says Firefox for Android does not synchronize extension storage.sync data with the user's account. Desktop WebExtension sync behavior should be tested among desktop Firefox instances.

Are synchronized extension records private from Mozilla?

Mozilla says Firefox Sync is end-to-end encrypted and Mozilla cannot decrypt synced content. That does not make every local extension record encrypted on disk or prevent the extension itself, another authorized browser instance, or a separate developer service from accessing data within its own design.

How can I tell where an extension stored my AI data?

Start with the extension's privacy policy and documentation. Advanced users can inspect extension storage through Firefox's debugging and Storage Inspector tools. Also compare Sync settings, the extension's account settings, exports, and network behavior. Do not infer the storage layer only from where the content appears in the UI.

Bottom Line

Firefox Sync can overwrite a newer local AI-extension edit when another desktop has already changed and synchronized the same key. The risk is most serious when an extension places many drafts, settings, or indexes inside one synchronized object and offers no conflict screen or restore history.

Use a synthetic two-device test, keep independent exports, and verify the extension's exact storage design. If you try OpenVeil, treat its browser-local normal chat history as a privacy and history boundary—not as a Firefox Sync backup or a substitute for records you must preserve.

Sources

When privacy, account control, uploads, and search matter, OpenVeil gives you a private AI workspace designed for that job.