This thread: https://support.mozilla.org/en-US/questions/1267353 may give you an idea of the Firefox files involved in the 'session restore' process. If they are stored in your backup (you may check for correct date), just follow the steps of the thread. good luck, Jürgen.

Hey thanks Jürgen The link you sent seems pretty windows-centric and I am working on a Big Sur Mac laptop... I wrote to jscher2000 who seemed to have solved the issue you linked to — perhaps he has solutions and if so I will post here. Thx and best, S

Note that on Mac closing Firefox can be more complicated because you can close all open windows, but keep Firefox running with the menu bar visible or minimized in the Doc.

Make sure to "Quit Firefox" with all windows still open. Otherwise check "History -> Restore Previous Session" and "History -> Recently Closed Tabs/Windows".

Location used for the main profile that keeps your personal data (Root Directory on about:profiles).

• ~/Library/Application Support/Firefox/Profiles/<profile>

In Mac OS X v10.7 and later, the "~/Library" folder in the Home directory is a hidden folder.

• https://www.macworld.co.uk/how-to/mac-software/hidden-files-mac-3520878/

You can try to locate sessionstore.jsonlz4 in the Time Machine backup and if that doesn't work you can check the sessionstore-backups folder.

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

Thank you cor-el! I think I follow but just to be clear for a non-pro like me: First quit Firefox (which I already quit once since making this error so keeping it open is no longer an option). Then in my time machine back up, I should locate all 4 bulleted files noted in sessionstore-backups folder. Then I should find whats currently in those sessionstore-backups on my machine now and move/ save them into a new folder (just in case) — and then I should replace those files with the time machine backup versions. Then on reopening Firefox i can restore old session? Thx and best S

Yes, hopefully that works if the backup has your lost tabs.

You can look at this tool to inspect a compressed sessionstore file.

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

Note that you need to place the file you want to use at the main profile level (i.e.where you see sessionstore-backups) and rename the file to sessionstore.jsonlz4 to replace the current sessionstore.jsonlz4 file.

Hey cor-el, just wanted to thank you again — this worked — so grateful for your help!