The exploited instance of IE7 probably spawns cmd.exe with the same
privilege levels as IE7 in Protected Mode, which means you don't have
read/write access to the user or system files. It's still bad because you
probably get to harvest all of the saved username/passwords in the browser
and capture all input/output from that IE session.
Now in the case of an exploited Firefox 2, you have full read/write
permissions to all of the user files which means you get to steal all the
user files and/or encrypt them for ransom.