And it has been already added to the default repositories of Ubuntu 14.04 Trusty Tahr, Ubuntu 12.04 Precise Pangolin, being available for both the two systems and their derivatives: Linux Mint 17 Qiana, Linux Mint 13 Maya, Pinguy OS 14.04, Elementary OS 0.3 Freya, Elementary OS 0.2 Luna, Deepin 2014, Peppermint Five, LXLE 14.04, Linux Lite 2.0 and others.
All you have to do in order to get Firefox 33 is to update the local repository index and install the firefox package. Like this:
$ sudo apt-get update
$ sudo apt-get install firefox