hello, maybe one of your addons is interfering here - however when the issue can't be reproduced reliably then it might be quite difficult to troubleshoot it...

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Inconsistent problems are hard. When extensions update, they sometimes open a new tab to inform you about changes. I wonder whether that is somehow failing halfway through the process?

I do have quite a few addons, I wouldn't be surprised if one of them is opening a new tab unsuccessfully. Is there a way to stop addons from opening tabs after an update? I would imagine that would be built into the addon, meaning "no". Of course, I can't check which addon isn't updating unless it updates, and the next update for the addon could fix this bug entirely. Although, if I could downgrade my addons, I might be able to figure out which developer to contact.

Another idea: I'll disable automatic updates for addons on my desktop for now. When my laptop next sees this bug, I'll backup the profile on the desktop, and update addons one by one. If I don't see this bug again in the next month, I'll call it fixed. (I see the bug every week or so, not really timing it.)

I haven't seen this bug in a while, I'm going to call it solved. My guess is it was an addon, and the developer caught on and fixed the bug in time for their latest version. I see that some of my addons haven't updated since I posted this bug report (some of them in the last year), so if I see the bug again, I'll just manually update addons on another machine until I can reproduce it. Then I'll tell the developer and post it here, if anyone's interested.