As a reminder, Firefox 38 has been recently released, bringing a new Tab-based Preferences menus, some special changes for developers, a bunch of HTML5 improvements and the autocomplete=off feature is no longer supported for username/password fields. (For more information, you can read the full changelog HERE.)
Recently, Canonical has added Firefox 38.0 to the default repositories of all the supported Ubuntu systems and their derivatives.
So, in order to get Firefox 38.0 on Ubuntu 15.04 Vivid Vervet, Ubuntu 14.10 Utopic Unicorn, Ubuntu 14.04 Trusty Tahr, Ubuntu 12.04 Precise Pangolin, Linux Mint 17.1 Rebecca, Linux Mint 13 Maya, Pinguy OS 14.04, Elementary OS 0.3 Freya, Deepin 2014, Peppermint Five, LXLE 14.04 and Linux Lite 2, you need to refresh the repositories:
$ sudo apt-get update
And install the firefox package:
$ sudo apt-get install firefox