Did you check "History -> Restore Previous Session" and "History -> Recently Closed Windows/Tabs" ? *https://support.mozilla.org/en-US/kb/restore-previous-session ---- You can use the button on the "Help -> More Troubleshooting Information" (about:support) to go to the current Firefox profile folder or use the <b>about:profiles</b> page (Root directory). *Help -> More Troubleshooting Information -> Profile Folder/Directory:<br>Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder *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 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 You can try to restore the upgrade.jsonlz4 file from the last update of if you use the Time Machine or other backup, try to restore file(s) from this backup. *https://support.apple.com/guide/mac-help/restore-files-mh11422/mac 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