I've had import export tools installed and I don't recall it throwing up a tab on every startup.

Have you contacted the authors of those addons?

I'm not sure this has anything to do with the add-ons themselves. Those same add-ons are installed and in use on about a dozen of our machines -- including this one -- with no problems.

I didn't mention it before, but I also tried starting TB in Safe Mode with add-ons disabled. It made no difference: The tabs still opened.

Session.json is supposed to be the file that records what tabs are open. In theory, deleting that file before starting TB should cause TB to forget about those tabs, but it still remembers them.

Let me rephrase my original question: What else, besides session.json, makes TB remember what tabs are open?

I think we need to see a screenshot of these "tabs" - no-one here seems to recognize what you have described.

Here you go. Both tabs say the same thing, differing only in the name of the add-on they want to install.

Zenos, thank you for prompting me to provide a screen-shot. Your request led me to a possible solution.

Reviewing my own latest reply made me wonder about the "Location" shown in the screen-shot window. Visiting that location revealed that the two add-ons that keep trying to install were .xpi files, loose in the Extensions folder. Then I wondered, what would happen if I deleted those two files and restarted TB? Deletion duly performed, and lo, the offending tabs are now gone.

However, those two add-ons now appear to be uninstalled...

The extensions folder seems to be the natural home for certain xpi files. I don't know the details, but it appears that some add-ons are expanded on demand, rather than permanently installed "into Thunderbird". It doesn't surprise me in the slightest that removing anything from this folder leads to the associated add-on being removed from Thunderbird.

Developers use phrases in connection with add-ons such as "restartless" and "bootstrapped", neither of which means very much to me, but it all goes to show that add-ons come in multiple flavours. I do know that some of these flavours will vanish with TB 60.

As an experiment, I re-installed Smiley Fixer (ver 1.4.3), and the problem came back. So Smiley Fixer has been removed yet again.

Interestingly, neither Import/Export Tools nor Smiley Fixer come up when searching for new Extensions within the TB UI; both of those extensions had been installed using files that had been downloaded at an earlier date. But then these same files had been used to install those extensions on all the other computers involved, and this machine is the only one that has a problem with them. I guess the user will now just have to get used to emails from Outlook users that have stray "J" characters in them.

I'm not going to bother to do any more experimentation, having been diverted from my real job for too long already, but I'm guessing that it's an iffy idea to install add-ons that are not offered through the TB interface.