Do you notice Firefox using the wrong file browser for other file system interactions, like Save Page As (Ctrl+S) or the Browse... buttons on this forum?

I'm a Windows person, but I'm aware of some file browser-related settings in Firefox for Linux. You could take a look here:

https://support.mozilla.org/en-US/questions/1386312#answer-1527471

In KDE Plasma, you can use System Settings to specify applications to use. Look at Personalise > Applications > File Associations > Inode : directory.

Thanks for your suggestions, guys. Sadly, they didn't help. Everything seemed to be set up correctly.

It turns out that the application kid3-qt (a tag editor) was the problem. I completely removed it, and now Firefox and Thunderbird are functioning properly again. Opening the profile folder in the correct file manager.

I should have realized it was the tag editor earlier. Still, we live and learn.

Thanks for taking the time to look at the problem; it is much appreciated.