Unable to install themes or extensions
From MozillaZine Knowledge Base
There are many reasons why this may be occuring.
Enable Software Installation
Tools -> Options -> Advanced -> Software Update -> Allow third-party sites to install extensions
Note that this will only allow sites to install extensions and themes and nothing more. You may want to disable this after finishing installation, as there are some spyware extensions circulating the Internet. These, to reassure, cannot be installed without first being granted permission via the "OK, Cancel" extension installation dialog, which has a two-second delay before you can select "OK".
Not Compatible
Be sure the theme/extension is compatible. Note that [Mozilla Update] will always have the latest, compatible versions.