How do I upgrade MAC OS X Lion to use the latest Mozilla Firefox?

I want to use the most secure system (Mozilla Firefox, etc). I have been using Firefox but the now it is no longer supported on my MAC OS X Lion 10.7.5. How do I upgrade? Is Mozilla Firefox still the most private and secure or is there an alternative that also doesn't require significantly more battery power? My data is backed up. (MacBook Pro Mid 2012)