Step 1 : Open firefox browser.If you donot have you can download it from here.
Step 2 : Now Download Stylish addon for firefox From here.
Step 3 : After installation is complete restart your browser.
Step 4 : Install Any of the themes From here.
Step 5 : To switch between the themes you have installed on your firefox browser go to Tools den select Addons option and choose Stylish and disable all other themes except the one you want.
Facebook is the most famous social networking site with awesome apps and games. But unlike orkut, facebook does not support themes. But you can install themes on facebook by just using a simple trick.
Installing Various themes :