I can't find just the Torbutton browser plugin for Firefox. I would like to find and install it so I can tor-ify Firefox so that I can easily toggle Tor if I need to quickly check Facebook at school (though that's what I have my VPN for...) It would also be nice to use it as an extra layer of anonymity!
The toggle model of Torbutton was discontinued a while ago. Mike Perry, developer of Torbutton, described the decision in a blogpost. So while you might be able to build a custom XPI and install it into Firefox, it is not recommended anymore.
The better solution is to use Tor Browser Bundle. Just start it from your USB drive or the hard drive and access the pages you want.