Does it still happen in [[Diagnose Firefox issues using Troubleshoot Mode|Troubleshoot Mode]]?

You can check for problems with the sessionstore.jsonlz4 file in the Firefox profile folder that stores session data. *rename/remove sessionstore.jsonlz4 You may also have to rename the sessionstore-backups folder. When sessionstore.jsonlz4 is deleted you lose pinned-tabs and open tabs and you will have to recreate them, so if necessary make a note or bookmark them. sessionstore.jsonlz4 is only present in the Firefox profile folder when Firefox is closed. You can use the button on the "Help -> More Troubleshooting Information" (about:support) page 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