O.K., I think I know what your problem is. But I can't see how to fix it.
The problem: You've got your default application for file management set to "Cervisa." This
used to be done in one of the KDE/GNOME "settings" options, but I can no longer find it.
I discovered this by creating a "Test" profile for Firefox and clicking on the "open folder" button. For me, that opened the folder in Emacs, which I realized was what I'd set the default to be back when I was using FC3. "Ah", I said, "I'll just change that, and all will be well." But I could not find the setting to change. I searched in both the KDE "Control Center" and the GNOME "Preferred Applications," but neither had the setting I needed to change.
(Of course, Emacs, unlike Cervisa, works fine, so its not a problem for me.)
Now I'd never noticed this in Firefox since the two extensions I mentioned "mask" the behavior. (And improve downloading.)
So, bottom line, that's you problem, but I don't know how to fix it. Sorry.
Oh, this may be a worse problem for you, since the "default manager" setting is used by other applications in addition to Firefox. Hopefully someone else will reply to this thread with a fix. (Oh, while I had Emacs open, I went through my home directory to see if I could find the setting in some hidden directory, but failed to find it.)