If you force a Firefox close then Firefox won't be able to write sessionstore.jsonlz4 in the profile folder. Maybe you have a upgrade.jsonlz4-<build_id> file in the sessionstore-backups folder in the Firefox profile folder that you can use instead. 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 can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the <b>about:profiles</b> page. *Help -> 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 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 sessionstore file. *https://www.jeffersonscher.com/res/scrounger.html ---- If you use the Time Machine or other backup then try to restore sessionstore.jsonlz4 from this backup. *https://www.apple.com/support/timemachine/ Location used for the main profile that keeps your personal data (Root Directory on about:profiles). *~/Library/Application Support/Firefox/Profiles/&lt;profile&gt;