Sorry to hear about this problem. Don't exit out of Firefox.

Is it possible to recover using either of these:

Restore Previous Session manually

• "3-bar" menu button > History > Restore Previous Session
• (menu bar or Alt+S) History > Restore Previous Session

Reopen Closed Windows manually

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

• "3-bar" menu button > History > Recently Closed Windows
• (menu bar or Alt+S) History > Recently Closed Windows

If any other data was missing at startup and was restored from the Sync cloud after signing in, you might look at the first section of the following article about checking other profiles: Recover lost or missing Bookmarks.

Please back up the existing session history files:

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 <= this might be the freshest backup?

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.

Whatever I tried to recover wasn't working and in the end I resorted to drag and drop the latest upgrade.jsonlz4-nn...nn to the following page:

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

This recovered the tabs I was after. The instance of firefox I was trying to recover tabs for did not have any collections and I have no idea if it would have found those tabs. I have another Windows laptop which has not updated yet so will bookmark all the tabs ensuring the collections are all open otherwise they will not be included in the 'select all' bookmarks which I will use to make sure I have all I want.