As if Firefox wasn’t a great tool to manage and enjoy the web, enter Personas for Firefox, an extension that enables you to choose over 50 themes (minus the icons) instantly with a click of the mouse.
Once installed, click the little fox mask icon on the lower left side of the window to select any number of themes without having to restart Firefox.
The icons and overall design of whatever theme you use stay the same (I’m using macfoxIIgraphite 2.1.4 on my portable Firefox), so only the address bar area of the browser changes scenery. This is the “Niagara” theme:
No restarts, quick changes, and you get to keep the icons and overall design of your favorite theme. Sweet!