If you're still on Mint 11 (guessing you are from your other recent thread), then it's probably not a major surprise that later versions of FF aren't in the repos. Is there a specific need to stick with an older Mint version?
This. Get on the latest version.
I suspect you'll still need to upgrade the base OS, but you can try the following:
Go to mozilla.org, grab latest version for Linux.
Untar.
Delete /opt/firefox, move the firefox folder that you got from the tar file into /opt.
It may work, but it may also have dependencies that you can't sort due to running an older distro.