For as long as I can remember, when I download something, I can click "open containing folder" from the "downloads icon" in FF, and the folder opens in Dolphin. Now for some unknown reason FF has decided to use a file manager "Files".... Which I did not even know was installed!

I found this when googling about and have followed the advice there, and the entries are added to the file /.local/share/applications/mimeapps.list, but FF still insists on using "Files" I have logged out and back in.