Well I've finally found it.
In the update page: http://www.mozilla.com/en-US/firefox/update/
Takes you to the step by step guide: https://support.mozilla.com/en-US/kb/Installing+Firefox+on+Linux
Firefox updates itself, we can check manually if there are updates: https://support.mozilla.com/en-US/kb/Updating+Firefox#Manually_check_for_updates
But in Ubuntu Firefox is automatically updated by the package manager, so the manual update will be disabled: https://support.mozilla.com/en-US/kb/Check+for+Updates+is+disabled
If that happen you will be lead to directions on how to install firefox on linux: https://support.mozilla.com/en-US/kb/Installing+Firefox+on+Linux
In this case we want up update from outside the package management:
That at the end tell us how up upgrade with the official instructions: https://edge.launchpad.net/+help/soyuz/ppa-sources-list.html
At this point I quit. I'm using Ubuntu because I don't want to deal with all this problematic steps of going from one page to another and add strange directions to simply install a product.
I guess I'll have to wait for the package manager to have the upgrade ( although If I understand well that will never happen, for I will only get upgrades in the 3.0 line )
Anyway, if anyone knows how to do something as simple as a double click on installer ( or much better, drag and drop on Applications as in OSX ) let me know, would you?