As you may know, Firefox 40 has been released a while ago, but now Canonical has updated the packages on Ubuntu all the supported Ubuntu systems to change the default search engine from Yahoo to Google and to fix a bunch of bugs.
Most likely, Canonical has forgot to replace Yahoo with Google as the default search engine, the Firefox browser for other platforms using Yahoo.
To get the latest version of the browser on Ubuntu 15.04 Vivid Vervet, Ubuntu 14.10 Utopic Unicorn, Ubuntu 14.04 Trusty Tahr and Ubuntu 12.04 Precise Pangolin (and derivative systems like Linux Mint 17.2 Rafaela, Linux Mint 17.1 Rebecca, 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 6, Peppermint 5, LXLE 14.04 and Linux Lite 2), you need to update the local repository index and install the firefox package:
$ sudo apt-get update
$ sudo apt-get install firefox