How to remove gimp association with pdf in Firefox

Quite easily, actually. Note, my only problem was that the Download manager of Firefox opened pdf-s with GIMP, otherwise, pdf-preview work ok.
So edit:
sudo nano /usr/share/applications/mimeinfo.cache
find the line starting with:
application/pdf=
and make sure it becomes:
application/pdf=kde4-okularApplication_pdf.desktop;
Ctrl+o + enter and you're done
Of course, you can change Okular to whatever else you use to read pdfs. But I like okular.
You can check the shortcuts in /usr/share/applications to see what other pdf viewers you have.
Cheers.