Aren't there recently closed windows available when this happens? *"History -> Recently Closed Windows/Tabs" Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox Title bar, especially if you have multiple windows or Private Browsing windows open to prevent losing tabs in unnoticed windows. *"3-bar" menu button -> Exit (Power button) *Windows: File -> Exit *Mac: Firefox -> Quit Firefox *Linux: File -> Quit You can check "History -> Restore Previous Session" and "History -> Recently Closed Windows/Tabs". *https://support.mozilla.org/en-US/kb/restore-previous-session ---- Firefox keeps upgrade.jsonlz4 files in the sessionstore-backups folder created when Firefox updates. Those are the only sessionstore files available when you can't restore lost windows via the history menu. You can copy such a upgrade.jsonlz4 file to the main profile level with Firefox closed as sessionstore.jsonlz4 to replace the file created by Firefox. Keep a backup copy of sessionstore.jsonlz4 and all files in the sessionstore-backups folder. You can look at this tool to inspect a compressed jsonlz4 sessionstore file. This tool works locally, no uploading is done. *https://www.jeffersonscher.com/res/scrounger.html