This problem has been covered in this forum before. However, the various solutions have not worked for me because it seems my backup previous.jsonlz4 was corrupted. I did exit Firefox normally, but I am not sure because I have been using multiple different browsers in recent months. This is what happened this morning: my Firefox on Mac OSX was closed normally, and I rebooted the MacBook. Afterwards, I could not restore old tabs because "Recently Closed Tabs" and "Recently Closed Windows" are greyed out. I tried copying recovery.jsonlz4 from the last folder and putting it into the main folder as sessionstore.jsonlz4, then reopening Firefox. Still have no restored tabs. Here was the surprise, which I do not know how to interpret. I just checked some files and found this error message in sessionrestore-logs folder: 1770888233416 SessionStore ERROR SessionSaver write() rejected with error: Error: SessionFile is closed(resource:///modules/sessionstore/SessionFile.sys.mjs:424:29) JS Stack trace: write@SessionFile.sys.mjs:424:29 write@SessionFile.sys.mjs:47:32 _writeState@SessionSaver.sys.mjs:375:29 _saveState@SessionSaver.sys.mjs:308:17 _saveStateAsync@SessionSaver.sys.mjs:354:10 saveStateAsyncWhenIdle@SessionSaver.sys.mjs:194:14 requestIdleCallback/<@Timer.sys.mjs:180:15 observe@AsyncShutdown.sys.mjs:570:17