Firefox themes can be enabled in the same menu that is used to enable and disable extensions. Click on Tools > Add-ons to go there and select Themes from the tab menu there. Firefox has to be restarted every time a new theme is activated.
1.
Microfox
2.
Gray Modern 2
3.
Curacao
4.
Scribblies Kids
5.
Phoenity Modern
6.
Red Cats
7.
Outlook 2003
8.
Noia 2 Lite
9.
Halloween
10.
Glorywine
11.
Abstract Classic
12.
Redshift V2
13.
Littlefox
14.
Metal Lion 300