Backup the session files in the sessionstore-backups folder in the Firefox profile folder to make sure you do not lose possible important session data. Do NOT close Firefox when Firefox is already running.

• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

You will normally find these files in the sessionstore-backups folder:

• previous.jsonlz4 (cleanBackup: copy of sessionstore.jsonlz4 from previous session that was loaded successfully)
• recovery.jsonlz4 (latest version of sessionstore.jsonlz4 written during runtime)
• recovery.baklz4 (previous version of sessionstore.jsonlz4 written during runtime)
• upgrade.jsonlz4-<build_id> (backups created during a Firefox update)

You can copy a file with Firefox closed from the sessionstore-backups folder to the main profile folder and rename the file to sessionstore.jsonlz4 to replace the current file.

• make sure to backup the current sessionstore.jsonlz4

You can look at this tool to inspect a compressed jsonlz4 sessionstore file. This tool works locally, no uploading is done.

• https://www.jeffersonscher.com/res/scrounger.html

Is Firefox set to restore your previous session automatically? If not, or if that didn't work as expected:

First, 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

By default, Firefox remembers up to 3 closed windows.

Also, about the older version:

Your Firefox reported that it is quite an old version (88 -- I guess you are not a fan of the Proton design?). If you recently downgraded from another version and had to create a new profile, you may be able to transfer your session history files from the other profile, but because file formats change over time, it's difficult to know whether those files will be compatible with Fx88. Keep safe copies just in case.

I forgot to mention:

If you close a window and realize immediately that it was a mistake, you can restore the closed window either from the menu I mentioned earlier, or with Ctrl+Shift+N. (In newer versions of Firefox, Ctrl+Shift+T may also do this.)

To exit Firefox without the risk of closing the wrong window, use the menu. Either:

• menu button > Exit
• (menu bar) File > Exit

All of the windows and tabs that are open when you use Exit will be restored when you restore your session, without having to look through closed windows.

I will lock your other thread about this issue.

• /questions/1464292 return closed tabs