I'm using Firefox 15.0.1 on Fedora 17 without running gnome or kde.
The problem I'm having is that whenever I select text outside of Firefox, for instance in xterm, the middle mouse button doesn't copy it inside Firefox, for instance in a text area, but rather brings up a context menu.
A related problem is that whenever I middle click inside Firefox, for instance in a text input, the middle mouse button brings up a menu when I'd like it to paste. Even if I select Paste in the menu not the selected text (from outside Firefox) gets pasted but the last selected text inside Firefox.
In about:config I tried "middlemouse.paste true" and also "middlemouse.paste false" together with the add-on Auto Copy but no combination worked. A middle mouse click always brings up a context menu. But the Auto Copy did help with automatically copying selected text to the clipboard. With Auto Copy the only problem I still have is pasting by middle button.
Follow up: somehow the problem solved itself. After removing Auto Copy firefox works as I expect it to (as any X application). I can't figure out why it was not doing it before, probably I was messing too much with about:config and not restarting frequently enough.