Thematic (was Persona Switcher) от drb80

Persona switcher adds a submenu to "Tools" that allows you to quickly switch between themes. It adds the following three user-definable key combinations.

• Shift-Alt-D to reset the theme to the default.
• Shift-Alt-R to rotate through your themes.
• Shift-Alt-A to start or stop switching between themes on an on-going basis.

You can customize the key combinations via "Manage Extension Shortcuts" in the about:addons page. There are several other ways to customize theme switching using the preferences.

If you're interested in a similar extension that allows the grouping of themes, check out: https://addons.mozilla.org/en-US/firefox/addon/vivida/.