You can upgrade your old firefox version to latest stable one using terminal. Even it is possible to use GUI, im gonna do on terminal, As many linux users me too familiar with terminal, less GUI.
follow these steps.

1. open Terminal then type commands step by step

2. sudo add-apt-repository ppa:mozillateam/firefox-stable

3. sudo apt-get update

4. sudo apt-get install firefox

Thats all, enjoy firefox..