Hi David Peebles, Select "Use Other" and a Select Application Helper window will open. If Okular isn't listed, select Browse and navigate to the executable file.

Many thanks for your rapid reply. That still leaves LibreOffice Draw as the default though. I'd like to remove that option entirely.

Do you mean that pdf's are opening in Okular but that LibreOffice is still listed?

Maybe this article will help you get it all sorted out:

Change what Firefox does when you click on or download a file https://support.mozilla.org/en-US/kb/change-firefox-behavior-when-open-file#firefox:linux:fx70

Yes, that's correct. I'd like to find the information in Firefox that sets the default apps for each file type. I can't find it from the article you sent but I'll keep trying. Thanks again.

Go to Menu -> Preferences -> General page scroll down to Applications

The information is in the 'handlers.json' file in the profile directory but it's encoded in some way that I haven't worked out yet.

I've found the solution. It's a problem with the file/app association system in Kubuntu. Although Okular is listed as the top default app, LibreOffice Draw is seen by Firefox as the default. When I delete LibreOffice Draw in Kubuntu's 'default apps' list for PDF documents, Okular becomes the default in Firefox.

That's strange. You should have been able to select Okular as the default in Firefox.

Anyway, good work hunting that down.

Browse Safe!

Yes, it seems that Firefox looks to the host OS for the default application (which I guess makes sense really). Thanks again for your help!