When you start typing into the address bar in Firefox you normally see a popup menu with all the URLs that your browser knows about. But what if you get tired of hitting the down arrow key every time, and would rather have it complete inline as well?
Type about:config into the address bar, and then filter by the following text:
Double-click on the value for “browser.urlbar.autoFill” to set it to true. The change should be immediate, no need to reboot Firefox… just start typing a previous URL into the address bar.
I’m still not sure if I prefer this or not, although it does seem to save time in some instances.