首先,移除iceweasel:
apt-get remove iceweasel
Then, download the latest Linux build of Firefox directly from Mozilla. Extract the files, and navigate to that folder, and run it. If you want, make an icon on your desktop. You can also make a link to the binary in /usr/bin/firefox to make it easier to launch.
If you want a package-based one, you can use Linux Mint's Debian package repo. To do this, add the following line to your /etc/apt/sources.list file:
deb http://packages.linuxmint.com debian import
And add the GPG key for that repo. Then, just run:
apt-get update
apt-get install firefox
And you should be good to go!
The default install is German, to install in any other language, you can manually run:
apt-get install firefox-l10n-en-us
Or use firefox-l10n-en-gb for British English.