Firefox normally creates a upgrade.jsonlz4-<build_id> file in the sessionstore-backups folder when it updates.

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

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

The people at the support desk do not understand that those of us who read these posts prefer simple, direct answers -- not several lines of previous.jsonlz4blahblah and recovery.jsonlz4blahblahblah. The reality is that I was deceived by Firefox to update to a newer version and there was NOTHING stated about do this and do that and do something else first in order to save your open tabs. Why would anyone want to update to the latest Firefox version when the previous one was working just fine? I would like to know whether I can revert back to the previous version because my experiences thus far with the newer version are very negative.

Yeah, that was a bit technical.

The larger context is: data loss during a routine update is never intentional. Something very unusual happened, and you may need to call on a backup file that Firefox creates during automatic updates. Or you may need to switch profiles.

Couple questions:

Do you update whenever Firefox has a new release (typically 2-3 times a month) or had you been holding off for a long time? If there was a big version jump, sometimes a incompatible file may be renamed without conversion.

Did you Firefox mention creating a new profile? To check for other profiles, try the steps in the "Determine if Firefox has created a new profile" section of this article: Recover lost or missing Bookmarks.

What security software do you have as sometimes security software interferes and prevents Firefox from updating files ?

• https://support.mozilla.org/en-US/kb/restore-previous-session