In supported versions of Ubuntu (Precise, Trusty and Utopic) it's sitting in both the updates and security repos. In Vivid (which is still just about in development, it's just in the main repo.
You either...
... have a stale package list (sudo apt-get update or click Update in your package manager)
... are subscribed to a [seriously] stale mirror of the updates/security repos. Switch to another using the Software Sources dialogue.
... aren't using a supported version of Ubuntu (Precise, Trusty, Utopic or Vivid). Upgrade.
Ubuntu 11.04 will release with Firefox 4.0. However, other supported Ubuntu releases will remain with Firefox 3.6 in the medium term. When Firefox 3.6 approaches end-of-life, the other supported Ubuntu releases will be upgraded to the latest version of Firefox, although this may not be 4.0.
For those Ubuntu releases, we will provide semi-official packages of Firefox 4.0 in a PPA.
Looking at the output of your sudo apt-get update command, it seems like you have a broken /etc/apt/sources.list file. To fix this, open Software sources by typing "software sources" in your dash and then check if there is a tick on the following:
Canonical-supported free and open-source software (main)
Community-maintained free and open-source software (universe)
Software restricted by copyright or legal issues (multiverse)
and the download takes place from the server nearest to you("Main Server" is fine for the moment but it may be busy at times).
And then open a terminal(Ctrl+Alt+t) and type the following:
sudo apt-get update
sudo apt-get install firefox
You should have the latest version of firefox now.
Best Answer
You can see from the full publishing history for Firefox in Ubuntu that v37 has been available in every supported version of Ubuntu in the hours following 2015-04-07 09:31:51 UTC.
In supported versions of Ubuntu (Precise, Trusty and Utopic) it's sitting in both the updates and security repos. In Vivid (which is still just about in development, it's just in the main repo.
You either...
sudo apt-get update
or click Update in your package manager)