Recently, a new build 13 Firefox (version tested) and is available for installation through the daily build repository. Since this is an unstable release and you may encounter errors when you use, so use it only for testing and curiosity. To install it, no need to remove the stable version, they can work in parallel without interfering with each other.
To install Firefox 13 in Ubuntu/Linux Mint , open a terminal, copy and run the following commands:
sudo add-apt-repository ppa:ubuntu-mozilla-daily/ppa sudo apt-get update sudo apt-get install firefox-trunk