Here is how to install Firefox 5 on Linux Mint / Ubuntu :
1. Open Terminal and first add the corresponding repository by typing :
2. Then update by typing :
3. Finally install :
During the installation, hit N when prompted.
All done, once install is complete, you should see Firefox 5 installed on Ubuntu / Linux Mint.
By the way, this whole three step process of adding a repository, updating and installing the needed package is applicable to all applications which are not recognized by default in Ubuntu / Linux Mint.
Happy browsing using Firefox 5 on Ubuntu / Linux Mint.