Yesterday Firefox 14.0.1 was released and the only way one could get it was to install it from external sources via PPA or direct download.
Now you don’t have to. All you need to do in order to get Firefox 14 in Ubuntu is to update your system, at least that what I did in Ubuntu 12.04 (Precise Pangolin). So, if you feel the need to upgrade, just run Update Manager in Ubuntu and you’ll get the latest Firefox version.
For more information about this release, click here.
Just run Update Manager and you should be able to upgrade Firefox.