You can check for problems with the sessionstore.jsonlz4 file in the Firefox profile folder that stores session data.

• rename/remove sessionstore.jsonlz4 and sessionstore.js

You may also have to rename the sessionstore-backups folder.

When sessionstore.jsonlz4 is deleted you lose pinned-tabs and open tabs and you will have to recreate them, so if necessary make a note or bookmark them. sessionstore.jsonlz4 is only present in the Firefox profile folder when Firefox is closed.

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Looks promising. However, in order to maintain several of my projects, the suggested procedure will require a temporary change to my normal workflow. Please bear with me while I make the necessary adjustments.

FWIW, after I filed the original problem description, I probed further and discovered that the "ancient" tabs were being restored in ALL four of the browser window I normally keep open, not just the frontmost window. In particular, two windows contained tabs whose dates were easily determined; and from these, I found that the tabs being restored were from more than a month before my last Firefox update. The tabs I should have seen were from 16-18 February; instead, the tabs that appeared were from 10-11 January—lost quite a bit of work. :-(

I'll change my workflow to accommodate your recommendations, then I'll report back with the results. Looking ahead and assuming your instructions work, please be thinking about whether there is a way to prevent this phenomenon in the future. As I originally reported, this has been happening with every Forefox update for at least the last several generations.