It is possible to install Firefox add-ons for all users that use a computer instead of installing them for every user separately. This is helpful if you are the computer guy in the family for instance and want everyone to have a basic set of extensions enabled that add functionality and security to the accounts.

The process of installing Firefox add-ons globally looks like the following. Download the xpi file instead of clicking on it which would install it directly. Once the add-on has been downloaded you use the following command at the run prompt (type Windows R):
firefox -install-global-extension

Let me give you an example:
firefox -install-global-extension c:\extension.xpi
The add-on will be silently installed and is available normally afterwards in the Tools > Add-ons menu of Firefox. Only administrators may uninstall the add-on but it is working for all users who are running Firefox.