Linux only: It usually takes some time for a new Firefox release to hit the official Ubuntu repositories after Mozilla releases it.
For the impatient out there, we present Ubuntuzilla. Ubuntuzilla is a python script that allows one to install the latest Mozilla products like Firefox, Thunderbird and SeaMonkey in a Ubuntu Linux machine. It can automatically download the latest Firefox and install it, integrate the already existing Firefox extensions with the new Firefox version and add/update menu entries in Applications -> Internet. It can also periodically checks for updated version of Firefox, Thunderbird or SeaMonkey and automatically download and install it.
Download the deb file appropriate for your system from Ubuntuzilla downloads page and install it (Installation instructions). Now, close all running Firefox instances and run
ubuntuzilla.py -a install -p firefox from a command prompt to start Firefox installation. Answer a few questions that the installer asks and you are good to go.
Ubuntuzilla is a free download for Ubuntu and Ubuntu derived Linux distributions.