Corrupted Debian Firefox Install
A long time back, I had issues with Mozilla's XUL files getting corrupted, then simple dialog boxes would no longer show. Instead, I'd get XUL XML parsing errors.
Firefox recently did it to me too. Watching the Debian user mailing list, someone else noted this, and they concluded that /tmp had run out of space during install, and the install script silently failed to build the files correctly. I cleaned up /tmp and tried again, and now it's fine.