%APPDATA%\Mozilla\Firefox\Profiles

It's possible that there is an additional file with the missing URLs. Could you start by making a backup of your existing session history files? Here's how: ''Do not exit Firefox, or if you closed it, don't re-open it.'' '''(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:'' Type or paste the following into the Windows Run dialog or the system search box and press Enter to launch Windows Explorer: %APPDATA%\Mozilla\Firefox\Profiles In that folder, do you see a semi-randomly-named folder? If so, click into it. If you find multiple such folders, find the one that was most recently updated. '''(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 * some .js files left over 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.