This userstyle emulates the suggested "Progress Line" in the 4.0 mockup of Firefox: https://wiki.mozilla.org/Firefox/4.0_Windows_Theme_Mockups#Combo_Stop.2FRefresh.2FGo_Button.

This style modifies Fission: it's required to make it work.

This userstyle was designed to be used in conjunction with the 3.7 mockup theme by BoneyardBrew at: http://boneyardbrew.deviantart.com/art/Firefox-3-7-Mockup-Redux-2-5-130502568 When/If he releases a 4.0 theme I'll add another userstyle suited for it. (The author has contacted me about integrating it into a 4.0 version).

You should be able to tweak it to fit other themes, fiddle with the margins.

Version 1.1:

Switched to base64 instead of the hassle of handling a .png thanks DMCrimson for the idea
Switched from background-color: transparent; to background: none; thanks for pointing it out IGLESIAS