Links open in Okular. How to fix?

I am running KDE Neon. Since a few weeks, links which in KDE or in non-KDE-apps I click on open in Okular, instead of my main browser Firefox. This began before the transition to KDE6. Of course, I have checked in settings - standard apps, if my browser is still set to Firefox. Yes it is. And text/html also links to Firefox. I am a little confused, if this is a bug or just a weird setting, which I have missed that I can configure it. Does anyone else have this experience or has an idea what I can do to fix.

I am most likely having the same or very similar issue. I was opening PO files from Firefox, PO files were set to be opened by POEdit, both in the system and in Firefox. From Dolphin, everything works fine, but in Firefox the PO files were opened in Kate, and no change fixed this. I have removed Kate, now it opens in Okular. If I remove Okular, it does not open anything, weird.

Edit: Upon further investigation, this is not case. It looks like that the download page serves PO files with text/plain MIME-type, which is indeed set to be opened either by Kate or Ocular.