I would recommend clearing the cache and cookies in your browser to see if there is old plug-in data interfering with the new plug-in. The link below should help out with doing this in Internet Explorer and Mozilla Firefox:
Hope this helps!
I just downloaded the auto-update Slingplayer plugin (22.214.171.1240/webslingplayer.xpi) for Moxilla and attempted to install it after accepting the Terms and Conditions.
It goes through Moxilla restart and again attempts to load Slingplayer; rinse and repeat - back to the Accept Terms and Conditions then reload and restart.
Any insight would be appreciated.