How to install Firefox 12 in Ubuntu 12.04 LTS

Use the program update-manager.

Click the button check; after a while all available updates(checked for stability ones) should be shown to you. With this also the firefox updates.

Pressing after this on install updtes(may be named different) and install all updates.

When you want to edit the automatic update behaviour of your system use the settings... button in the left down corner of update-manager.

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Thing on the canonical also offer support(that is better for newbies):

http://www.canonical.com/enterprise-services/ubuntu-advantage/desktop