I use the address bar in Firefox both to type URLs and to search. Firefox shows a dropdown with suggestions as I type. Seeing that the top result is what I want, I press Enter. Instead of going to the URL it showed as top result, Firefox instead goes to the partially typed text and tries to open or search for that! This is an irritating behavior that goes against how every browser address bar aid the typing user.
Not surprisingly, I found that this is not a Firefox behavior. Ubuntu installs some extensions to Firefox by default, involving Unity integration and other UI tweaks. Disable these Ubuntu extensions, restart Firefox and your addressbar will work without any hiccups.
Tried with: Firefox 40.0.3 and Ubuntu 14.04