Hi archivist66, do you have any session-related add-ons? If so, you might consider disabling those first before doing further testing. ''Built-in Session Restore Files'' Firefox normally updates the session history files as often as every 15 seconds, and then rearranges files at shutdown. Let's see whether that is happening as expected, which would suggest something unusual is happening during startup. First, 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" button. In your profile folder, double-click into the '''sessionstore-backups''' folder. 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) -- should be updated as often as every 15 seconds * recovery.baklz4: a backup copy of recovery.jsonlz4 -- updated with the previous version 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 Q1: Is recovery.jsonlz4 updating as you close tabs? Keeping that window open, switch over to Firefox and do a normal exit. Q2: Did recovery.jsonlz4 update after shutdown? If it is present, please make a backup copy: right-click > Copy, then right-click a blank area > Paste. If it is missing, simply go to the next step. Q3: If you navigate back to the main level of the profile folder, did Firefox create sessionstore.jsonlz4? Please make a backup copy: right-click > Copy, then right-click a blank area > Paste. To preview the contents of the files (use the backup copies), you can drag and drop it onto my page here, then click Scrounge URLs: https://www.jeffersonscher.com/ffu/scrounger.html Does that show the closed tabs are listed among the open ones?