Fission , developed by zeniko is a clever Firefox extension that merges several status bar functions with the address bar. For starters, it inserts the progress bar as a temporary background for the location bar. After using it for some days I realized how much having the progress more visible eases the wait for some slow loading pages.

Fission includes options to customize the progress bar colors or even choose a backgroud images. It can also display the current action (looking for, connecting, waiting, transferring data, done) and show the address for links in the current web page when you hovered over them (just like the status bar does).

A side benefit is that unless you have some other extensions that use the status bar you can hide it and save some screen for web contents. According to Firefox plans, it could become standard behavior of a coming official release. Get it at Firefox Add-ons.