Look on your desktop. Do you see a folder called; Old Firefox? Look inside. Look for the folder with the latest creation date.

https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

Thanks, there really was such a folder. FF still needs to learn not to kill my custom search engines (including not killing them when I merely change FF's language) and I still don't understand why FF would move my old profile data onto the desktop, but at least it's working again.

Can the team please debug those problems?

I think bug reports were filed.

Sorry, I don't understand what you mean by "think". Are threads in this forum filed every once in awhile, but you can't say for sure if and when they're filed? Should I file it somewhere?

The people who answer questions here, for the most part, are other Firefox users volunteering their time (like me), not Mozilla employees or Firefox developers.

If you want to leave feedback for Firefox developers, you can go to the Firefox Help menu and select Submit Feedback... or use this link. Your feedback gets collected by a team of people who read it and gather data about the most common issues.

Ok, thanks, I put the link to this thread into that form. :-)