There are source level changes to FireFox, thanks to Yawning for correcting this here. I'll add a caveat to this answer.
I previously believed the FireFox in TBB differs from stock FireFox though FireFox configuration, add-ons, and add-ons configuration. We can explore the add-ons :
At minimum, you should install HTTPS Everywhere because malicious exit nodes can sniff traffic leaving the Tor network.
Additionally TBB comes with a specially configured NoScript. I'd install NoScript but copy over the TBB's NoScript config files. Afaik, TBB's NoScript is much milder than default NoScript, so that TBB works with sites that NoScript breaks.
Finally, TBB has a Torbutton extension that "disables many types of active content", which sounds worth installing. I donno if Torbutton merely adjusts NoScript's configuration or if it plays a more active role.
You might read Google Chrome Incognito Mode, Tor, and Fingerprinting too because that describes shortcomings in the Chrome browser that prevent the TBB extensions from working securely enough. I've just asked about using Chrome through a web proxy on Tor.SE, which might net some interesting answers too.