Description of problem: Firefox is missing HiDPI support in Gtk2/Gtk3, see https://bugzilla.mozilla.org/show_bug.cgi?id=674373
Upstream has a good fix for that - https://bugzilla.mozilla.org/show_bug.cgi?id=975919. I added the patch to the latest rawhide build for testing (firefox-35.0-5.fc22).