Well, this is probably more of an issue for Mac Users (like me) using the Flip4Mac WMV plugin to play windows media. Apparently, every time I wanted to listen to Sirius online, it worked with no problem but that wretched alert kept popping up.
So what did I do? I went to mozilla.com and went to support and found a simple and great solution I did not know about. This may be no news to some but for those like me check out the next link.
Basically, you type about:config in your address bar and a whole bunch of configuration options pop up that you can set. Look for one called plugin.default_plugin_disabled and click on it to set it to false. That is it, restart firefox and voila! Wretched alert is gone! Remember though, if you do this you will not get that alert for any plug-in, but what the hell, you should know what you are opening up anyway.
Hope this helps someone. Firefox Rules, Safari is Great, IE needs an execution.