I'd love a progress bar on desktop Firefox. The current spinny/bouncy loading indicator can only indicate that something's happening, and not how long there is to go etc.
Firefox for Android has one:
Safari on desktop had/has one:
I like the Safari idea above, it combines the address bar and progress bar so you don't sacrifice any space. It also make smart use of something that you want/need to see at all times (the address bar), unlike the Find bar or the status bar which you want out of the way when not in use.
I remember a browser, might've been Opera, even showing the amount of data (eg 7MB of 14MB) which was cool too!