sessionstore.js would be from Firefox 55 or earlier. Since Firefox 56, files are compressed using lz4. Firefox creates numerous session history files, but because session history is only for the immediately previous session, it is unfortunately too easy to lose it. Could you start by making a backup of your existing session history files? Here's how: '''(1) To open your profile folder...''' ''If Firefox is still running:'' You can open your current Firefox settings (AKA Firefox profile) folder using either * "3-bar" menu button > "?" Help > Troubleshooting Information * (menu bar) Help > Troubleshooting Information * type or paste about:support in the address bar and press Enter In the first table on the page, click the "Open Folder" (or "Show in Finder") button. ''If Firefox is closed:'' See: [[Profiles - Where Firefox stores your bookmarks, passwords and other user data]] '''(2) Copy out session history files''' In your profile folder, double-click into the '''sessionstore-backups''' folder. Save all files here to a safe location such as your Documents folder. '''(3) What files did you find?''' The kinds of files you may find among your sessionstore files are: * recovery.jsonlz4: the windows and tabs in your currently live Firefox session (or, if Firefox crashed at the last shutdown and is still closed, your last session) * recovery.baklz4: a backup copy of recovery.jsonlz4 * previous.jsonlz4: the windows and tabs in your last Firefox session * upgrade.jsonlz4-''build_id'': the windows and tabs in the Firefox session that was live at the time of your last update * ''various'' .js files from Firefox 55 or earlier Could you take a look at what you have and the date/time of the various files to see whether you think any of them would have the missing tabs? To preview the contents of a file, you can drag and drop it onto this page, then click Scrounge URLs: https://www.jeffersonscher.com/ffu/scrounger.html That tool is on my site, so please let me know if it doesn't work for you.