Mozilla Firefox comes in a few different versions, depending on how new you want yours to be. There is:
Old-Hat Firefox – For people who do not like to upgrade their software. These are the versions that are years old. This stuff is OLD. Historical purposes only. Rising software security scientists might be interested in testing legacy exploits against this old browser. Because historical exploits are excellent sources to learn from.
Now-Firefox – What Mozilla says you should be using. People who update their systems when they should will be running this. Has the best stability of the gradient. If you want to browse the Internet and parse HTTP protocols and do all that normal stuff, this is where you should be.
New-Firefox – The stuff you read about in the technology publications. “Mozilla to release version Y.XX” and “New Firefox slated to be twice as slow and three times as memory hungry.” That sort of rot. Not a bad thing to use if you would like to help Firefox out. Download, install, and then run along. Be forewarned, it is new, so some extensions will not work and some websites will break.
Bloody-Edge-Firefox – For developers. This is that steaming pile of code that someone just dropped off minutes ago. No, really. This is updated every other minute and the people using this version are always compiling. Not for people who want to spend more time browsing than watching “make flags” fly by.