I'm glad to see work being done on TorButton. I was the person who came up with the modified TorButton idea with Mike Perry on IRC a long time ago.
I see at your site (http://korykirk.com/GSoC/tor_app.html) you mention Refspoof. I have emailed Mike Perry about this before. I was also the person who suggested to use a referring spoofing Firefox plugin. But not Refspoof. I emailed Mike about this because on the TorButton page referrs to Refspoof. The plugin I oringally informened Tor team about, and the one they refer to, is really "RefContorl" (https://addons.mozilla.org/en-US/firefox/addon/953), not Refspoof. I informened Tor team how RefContorl will spoof the root of the site you are visiting as the referrer. I do not know if Refspoof has this feature. To the feature it in RefControl is really easy. You should look at the code of RefControl, not Refspoof.
I also have a dangerous bug report to tell you about.. If you set Firefox/TorButton to start as disabled and you are downloading a file with Tor and Firefox crashes when it restarts it will continues downloading the file *without* Tor!
I am on Windows XP sp3 with current Tor-rc.
I'm loooking forward to you work!