In a second reply, I will mention an add-on to investigate what is stored in the Mozilla Sync cloud, but the best hope lies in the files Firefox creates on your computer.

When you restarted Firefox after the crash, did it mention any issues such as files being inaccessible or needing to create a new profile? If so, that will change the following steps.

If not:

Usually Firefox will attempt an automatic crash recovery for a regular (non-private) session.

Could you check whether either of these are available (not hidden or grayed out):

• "3-bar" menu button > History > Restore Previous Session
• (menu bar) History > Restore Previous Session

If they are missing or grayed, check either:

• "3-bar" menu button > History > Recently Closed Windows
• "Library" toolbar button > History > Recently Closed Windows
• (menu bar) History > Recently Closed Windows

Note: Each window also has its own Recently Closed Tabs list.

If that doesn't help...

Firefox creates numerous session history files, but because session history is only for the immediately previous session, it is unfortunately too easy to lose it. Could you start by making a backup of your existing session history files? Here's how:

Do not exit Firefox, or if you closed it, don't re-open it.

(1) To open your profile folder...

If Firefox is still running:

You can open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > Help > More Troubleshooting Information
• (menu bar) Help > More Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, find the Profile Folder row and click the "Open Folder" button. This should launch Windows' File Explorer listing the contents of your current profile folder.

If Firefox is closed:

Type or paste the following into the Windows Run dialog or the system search box and press Enter to launch Windows Explorer:

%APPDATA%\Mozilla\Firefox\Profiles

In that folder, do you see a semi-randomly-named folder? If so, click into it. If you find multiple such folders, find the one that was most recently updated.

(2) Copy out session history files

In your profile folder, double-click into the sessionstore-backups folder. Save all files here to a safe location such as your Documents folder.

(3) What files did you find?

The kinds of files you may find among your sessionstore files are:

• recovery.jsonlz4: the windows and tabs in your currently live Firefox session (or, if Firefox crashed at the last shutdown and is still closed, your last session)
• recovery.baklz4: a backup copy of recovery.jsonlz4
• previous.jsonlz4: the windows and tabs in your last Firefox session
• upgrade.jsonlz4-build_id: the windows and tabs in the Firefox session that was live at the time of your last update

Sometimes you can tell from the last modification time which one will have your missing tabs, but not always.

Optional File Contents Preview

These compressed files are a pain to view, so I created a tool on my website to list out their contents. If you want to try that, you can drag and drop it onto the large box on the following page, then click the "Scrounge URLs" button:

https://www.jeffersonscher.com/ffu/scrounger.html

If you don't get a list within 15 seconds, that probably means the script is caught in a loop. You may need to close the tab to avoid a tab crash and then try again in a new tab.

If you get a useful list, use the "Save List" button to archive it as a web page of clickable links for future reference, in case no other approach is successful.

You can check what data is in the Sync cloud for your Mozilla Account using the About Sync add-on:

https://addons.mozilla.org/firefox/addon/about-sync/

After you install the add-on, you can type or paste about:sync in the address bar and press Enter to load it. The page shows each category of data currently found in the Sync cloud.

Scroll down to tabs and look at the little tab that says "Records (object)" to see a quick count of how many tab titles and URLs are stored on the server (expand triangles as needed to see the tabs: line).

Anything there? Are your old tabs listed under a separate device from your current set of tabs?

Thank you for this detail, may be helpful going forward if I keep using FF but unfortunately the browser hard-crashed and did not prompt me to restore tabs, and as noted above the Sync feature did not work either despite being active. To confuse matters, I had two instances open, and only the one with far more tabs open crashed.

Your helpful website tool allowed me to manually restore from two weeks ago, which helps some. Thank you again!

When you say "two instances," if you mean two windows, use this:

• "3-bar" menu button > History > Recently Closed Windows
• "Library" toolbar button > History > Recently Closed Windows
• (menu bar) History > Recently Closed Windows

You can open about:sync-log via the location bar to see if recent Sync logs are available. You can upload the file to a website like Pastesite if you can't post the content of the file in a reply.

• https://pastesite.org/

In the past there was a maximum payload for syncing tabs, I don't know if this is still the case since in the past synced tabs also expired after some time.

In the past there was a maximum payload for syncing tabs, I don't know if this is still the case since in the past synced tabs also expired after some time.

When I check about:sync on a desktop PC, it shows 220 open tabs on Firefox Nightly for Android. Obviously I've been very lazy about closing them, but setting that aside, if there is a limit, it's pretty high.

See:

• https://searchfox.org/mozilla-release/source/services/sync/modules/service.sys.mjs#780

See:

• https://searchfox.org/mozilla-release/source/services/sync/modules/service.sys.mjs#780

thank you - does this mean tab limit is 256?

I think it means the limit is however many tabs will fit in 256KB (about 262,000 characters). Some sites like Google Maps have insanely long URLs, but most are short.