I'm not sure I understand. Are you asking whether Firefox supports browser extensions? If so, the answer is yes. Firefox has always had extensions including epic breakthroughs such as Adblock (Plus) from 2002 and Firebug from 2006. (Google Chrome has had extensions since 2010.)
> If I need the whole URL visible at all times as a developer, that's an hour of chrome extension writing to make it always viewable.
Interesting. Other comments in this thread indicate that the only extension that can do this has its ID hard-coded in the url parsing code to be whitelisted.
An extension can get window.location and can then paint it anywhere on the page the developer chooses, or put it in a drop down, or send it to an external service via an HTTP request and beam it to the moon, if the developer wishes.