Setting the default application in the "you have chosen to open" dialog box: as of Firefox (Iceweasel) 20.0, in ~/.local/share/applications/mimeapps.list, add a line like this to the Default Applications section:

application/pdf=kde4-okularApplication_pdf.desktop

This overrides the settings in /usr/share/applications/mimeinfo.cache, which is autogenerated by update-desktop-database. So now you have to manually sync your local mimeapps.list with mimeinfo.cache if you just want to change the program that is selected by default in the "you have chosen to open" dialog box. Dumb. I like to use Firefox's built-in PDF previewer most of the time, but when I want to download a PDF, I want to have Okular selected to open it by default, not GIMP.

Here are some Firefox/iceweasel config items I like to set:

Addons that I sometimes like to install: