Firefox Sync: Everything You Can Sync (and What to Do When It Breaks)

Firefox Sync is underused. Most people turn it on for bookmarks and forget about it. But there's more to know — especially when it breaks in subtle ways.

What Firefox Sync Actually Syncs

Firefox Sync can keep in sync across devices:

• Bookmarks — including folder structure
• History — pages visited on any synced device
• Passwords — via Lockwise/built-in password manager
• Open tabs — other devices' tabs accessible from desktop
• Add-ons — extension list (not all settings)
• Preferences — many about:config values
• Address bar / search — search engine choices

What it doesn't sync:

• Extension-specific data (each extension decides what to sync via its own storage)
• Cookies and active sessions
• Profiles themselves

Setting Up Firefox Sync

1. Go to about:preferences#sync or hamburger menu → Sign In to Sync
2. Create a Mozilla account (free, just needs email)
3. Choose what to sync — you can deselect categories you don't want

On mobile (Firefox for Android/iOS), sign into the same account and it works automatically.

Accessing Other Devices' Tabs

This is the most useful underused feature. With Sync enabled:

• Desktop: address bar → click the tabs icon in the right side, or about:firefoxview
• You can see open tabs on your phone from your computer (useful for "I was reading this article on my phone")
• Works in both directions

Common Sync Problems

Changes Not Propagating

Firefox uses a push sync — changes should appear within seconds. If they don't:

1. Check the Sync panel in Preferences — look for error messages
2. Go to about:preferences#sync and click "Sync Now"
3. Check whether you're signed into the same account on both devices

"Sync is disconnected" Error

Usually means the device token expired. Fix: sign out and back in. You won't lose data — it re-merges on next login.

Duplicate Bookmarks After Sync

This happens when you import bookmarks on a new device that already has some. Firefox merges by URL but creates duplicates if the same bookmark exists in different folders. Fix: check Bookmarks → Manage Bookmarks → All Bookmarks and delete duplicates.

Passwords Not Syncing

If passwords from another device aren't appearing:

1. Ensure the same Firefox Account is logged in
2. Check about:logins — passwords should appear after a Sync Now
3. If passwords were saved in an old format, they may need manual export/import

History Sync Gap

History sync has a default limit — older history may not transfer. This is by design to keep sync storage manageable.

Privacy Considerations

Mozilla encrypts Sync data end-to-end using a key derived from your password. Mozilla says they cannot read your synced data. Your password is not stored server-side — only a hash is kept.

If you want zero-trust Sync: you can generate a Sync Key (a random string) that provides an additional layer. Set this up in about:preferences#sync → Manage Account → Recovery Key.

Sync for Tab Management

One limitation of Firefox Sync: it syncs which tabs are open, but doesn't let you save named sessions or restore a specific set of tabs from a past date. For that, you'd want a dedicated extension.

Tab Manager & Session Saver fills this gap — it lets you save named snapshots of your current tabs (like "work sprint", "research project") and restore them independently of Sync. The saved sessions are stored locally as JSON.

This complements Sync well: Sync handles cross-device, Tab Manager handles "I want to save this exact set of tabs and come back to it".

Sync vs. Firefox Account vs. Mozilla Account

Quick terminology note that trips people up:

• Firefox Account → now called Mozilla Account (same thing, rebranded ~2023)
• Firefox Sync → the sync feature, powered by Mozilla Account
• Firefox Relay → email masking product, separate but uses same account
• Mozilla VPN → separate paid product, same account

One account covers all of these.

Advanced: Sync Server

If you want full control, Mozilla publishes the Sync server source code. You can run your own Sync server and point Firefox at it:

1. Set services.sync.tokenServerURI in about:config
2. Run the server at github.com/mozilla-services/syncserver

This is an option if you want your sync data hosted on infrastructure you control rather than Mozilla's servers. Overkill for most, but the option exists.

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Firefox Sync is solid for its core use case. The main thing to know is that extension data doesn't sync (each extension owns its data), and for tab session management beyond what Sync offers, a dedicated session saver extension covers the gap.