Firefox has long been able to handle themes. In general I’ve found them to be outlandish or annoying. I really want to get rid of those little down arrows next to the forward and back buttons, but I digress.
Some of the Mozilla devs thought themes were too much work to find and install, so they wrote up something called Personas. Personas are like micro-themes, or “light” theming. It doesn’t change everything in the browser, just the background of the toolbars and the status bar, and changes some font colors in the title bars if it needs to. It also makes the tabs semi-transparent.
One of the coolest parts about this is the ease with which you can choose and use themes. Once you’ve installed the extension there’s a small icon in the status bar. You single click it and a menu comes up with theme categories. You simply choose one off the list and it changes, no restarting of the browser.
That menu populates itself, so there’s no “going hunting” for themes. It’ll remember the last three you used, so if you try one and hate it, you can switch back easily.
I’ve been playing with it today, and really enjoy it, give it a shot!