Windows 7 default programs won't let me choose Firefox as my default browser
Do you have Firefox installed?
Ok, now that that's out of the way... Try this after exiting Firefox completely:
firefox.exe -silent -setDefaultBrowser
That will set firefox as the default browser and you should be good to go.
Maybe the tool I found on this page (Register Firefox Portable With Default Programs in Windows Vista) could be helpful.
I guess it should work for the regular Firefox version too.