Since I started to use Firefox, there is one thing that kept bothering me. When you click with mouses middle button on empty places, Firefox will try to open it as a URL. It seems to be very convienient when you copy a URL somewhere else, then you switch back to Firefox and press middle button. However, that is really rather used. Most of time, just Address Not Found.

So, open about:config, type middlemouse.contentLoadURL, then set it to false.