I just stumbled across a Mozilla Labs project called Personas. It’s light-weight theming for Firefox that can be changed without restarting the browser. After you install Personas, you get a new menu entry Tools->Personas for Firefox, where you can quickly change the persona you are using. From what I can tell, Personas seem to change the your browser toolbar and menu font colors and usually add a lightweight background picture. According to the website, the project has been going since Dec 2007, so there’s a lot of Personas to choose from.