Most of you may know this already but I just stumbled across it today when looking for a tweak to allow me to "paint" and select a portion of the URL in my FF address bar "quickly" like I can do in IE.
If you go to your address bar and type about:config there are hundreds of settings you can access. Kinda neat. Now lets go see how much I can mess things up!
EDIT: BTW, I think I found it. Cool.
browser.urlbar.ClickSelectsAll
Change it to false.