? Where are you getting your version from? I just checked the Mepis 11 and Debian Stable repos and am definitely running the latest one they have.
It’s automatically updated through lINUX MINT’s Update Manager.
You could also click on the ‘about Firefox’ link under the ‘help’ tab and upgrade that way too.
Linux Mint is based on Ubuntu, Debian, so I don’t understand why you shouldn’t be able to upgrade.