Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox Title bar, especially if you have multiple windows or Private Browsing windows open to prevent losing tabs in unnoticed windows.

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

You can try to inspect/restore the upgrade.jsonlz4 file created with the last update.

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

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> (backup created during an upgrade of Firefox)

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

• 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 done.

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

I can't find any of those files and I searched the entire Mozilla Firefox folder. I just got all of my tabs from that session to come back by clicking the 4 bars in the upper right-hand corner, then History, Recently Closed windows, Then clicked Reopen all windows. I would suggest that you guys implement a fix for this in the future where closing your main session while a Private Browsing window is open doesn't wipe your main session when you open Firefox again. Hopefully this helps someone else experiencing the same issue.

With Firefox closed, try these steps. see screenshots

Oh ok thx. I'll definitely take note of this and try it if I (god forbid) need it in the future. :)