Firefox users have access to a selection of VPN add-ons for the browser which they can install from Mozilla's official add-ons repository.
Whenever a connection to a virtual private network is established, it will be used as the entry point to the Internet. Websites and services that users connect to will reveal the IP address of the VPN server and not the local one.
That's great for a number of purposes. Since you connect to a server in a specific location, you can access contents that are restricted to that location.