Here's this lovely (read as aesthetic) extension for Firefox. Its called Fusion that lets you double up your location bar as your progress bar.
In case you dont like the default color it uses, you can always tweak your firefox display settings file (userChrome.css) to select you own color. Here's my setting (with important part marked blue):
#urlbar-progressmeter > .progress-bar { background-color: rgb(255,222,222) !important }
For more details on how to tweak other firefox settings, see Firefox Help: Editing Configuration Files.