This may or may not be possible. Could you start by making a backup of your existing session history files? Here's how: You can open your current Firefox settings (AKA Firefox profile) folder using either * "3-bar" menu button > "?" button > 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 "Show in Finder" button. If Finder shows a single folder rather than a long list of folder and files, double-click into that folder. Then scroll down and double-click into the '''sessionstore-backups''' folder. Save all files here to a safe location such as your Desktop. '''What files did you find?''' The kinds of files you may find among your sessionstore files are: '''''Firefox 56+''''' * 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 the Firefox 55 days '''''Older Firefox''''' * recovery.js: 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.bak: a backup copy of recovery.jsonlz4 * previous.js: the windows and tabs in your last Firefox session * upgrade.js-''build_id'': the windows and tabs in the Firefox session that was live at the time of your last update 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/res/scrounger.html That tool is on my site, so please let me know if it doesn't work for you. You also can save the page of clickable links as a backup.