Go to Applications > Ubuntu Software Center from the top panel go to Edit > Software Sources and click the ‘Other Software’ tab.Press ‘Add’ button and then paste the following line
System>Administration>Software Source> click TAB “Other Software”>Press “Add” button then paste below :

ppa:mozillateam/firefox-stable

after that… you can upgrade via GUI.. like this head to System > Administration > Update Manager
or you can install via terminal busway.. lol haha

1.    sudo add-apt-repository ppa:mozillateam/firefox-stable

2.   sudo apt-get update

3.    sudo apt-get upgrade

ok absolutely its right.. but i was try.. and succesfully… thanks to ubuntu-geek