I don't know when exactly it started doing it, it wasn't doing it in the beginning for sure. What happens is that when you click on a link to a pdf file, Firefox asks you what to do with it - open with, save file, etc. Since a couple of days "open with" defaults to gedit, instead of evince. And it's really annoying, as even if you manually select /usr/bin/evince , the next time you try to open another pdf, Firefox keeps offering gedit as a default pdf viewer.

Anyway, I don't know what is the reason for this, but here is the solution that worked for me, I hope it is useful for others as well:

Go to ~/.local/share/applications/ folder and check its contents. If you see the files mimeapps.list and defaults.list - delete them or move them elsewhere. Restart firefox and voila. If you're curious - check the contents of these files to try to find out what have put them there.

P.S. This is openSUSE 12.1 x64 with Gnome Cinnamon