This is no longer an option to restore previous session?

   Open Firefox and click on the “menu” icon on the right corner of the screen.

   Scroll down and select “Library” and then “History”.

   Select “Restore Previous Session” to restore your tabs.

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 use "Quit Firefox" with all windows still open, especially if you want to use "Restore Previous Session". Otherwise check "History -> Restore Previous Session" and "History -> Recently Closed Tabs/Windows".

If you lost the tabs due to using an extension then you need to contact its developer for support.

You can check if you have an upgrade.jsonlz4 in the sessionstore-backups folder that has the lost tabs.

• 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> (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

Don't quit Firefox yet. Can you use the following menu to recover some closed tabs:

• (menu bar) History > Recently Closed Tabs
• menu button > History > Recently Closed Tabs

The tabs might not be added back to their groups, but hopefully you can get the last 10-25 back.