To use a Firefox theme, you need to apply it after installing. Firefox 3.x will automatically apply the installed theme for you. If you are using version 2.x, do the following:

• Open the Themes Manager (in Firefox 2+ it's Tools - Add-ons - Themes, in Firefox 1.x it's Tools - Themes)
• select the installed theme
• click the Use Theme button

Now it's time to restart Firefox and enjoy your new toy...eh, theme. :)

To install a Firefox theme offline, drag the downloaded JAR file on the Add-ons Manager window. Note: do NOT double-click the file! You will end up either getting error message or opening the archive.

Big thanks to Pcdingo for beta testing. :)