well there is a work around and it is this:
Open a new tab.
In the address bar type about:config.
You get a screen that says This might void your warranty. Click I'll be careful, I promise.
Scroll to the bottom and look for xpinstall.sigantures.required. faster just search on xpinstall.sigantures
It will be set to True default. Double click on it to change to false. Restart your browser and you should be able to access Slingplayer
pretty simple, one would expect the rocket scientists to know?