The plugin installer can't detect the browser installation directory. Or, if Firefox is already installed on the host machine, it will likely install the plugin to the regular Fx installation.
http://www.microsoft.com/downloads/MozillaValidation.aspx?familyid=0a9b6...
There is an alternate validation method (browser independent), so this is just a heads-up.
Oh, and the alternate program MUST be run from the same drive as Windows.
So, install locally and copy it in.
Incidentally, this is really only for office environments to connect to secured internal sites which probably will only work with IE anyway.
Sometimes, the impossible can become possible, if you're awesome!
It's used to check Windows for authenticity. I needed to validate Windows when I wanted to download the latest DirectX form the MS website. I just wanted you to add this little bit of info to the FxP page to let users know. It's not really a problem.
Vintage!