Ok every FPS fan should kno QuakeLive, its free version of Quake 3 Arena + mods and enhancements, but it's browser based and sadly not only that u have 2 download the plugin for specific browsed, but even more sadly that it saves all the game itself in user profile folder like "C:\Documents and Settings\username " and so on.
but I've used different portable apps and kno dat some of em do this wierd trick - when u browse through windows to " c:\program files " u dont see the portable app that u are using, and yet when u browse through the portable app u see somthing like - well don't remember but maybe " c:\program files\Gimp " or maybe " c:\program files\nero9\burning ROM ".
And so maybe do the same thing with " firefox portable " and make it think that documents and settings are in the portable app folder, probably with portable app launcher.
I am not sure about the plugin tho !
Sorry i might not have been clear !:(