My ASPX page hosting my Silverlight 4 control was working perfectly well within IE8 but when I load the same page in FF (3.6…) I just kept getting the nice MS logo asking me to download and install Silverlight. I knew the SL add-on worked in FF because I had tried other applications.
As it turned out the problem was my initParams value which was initially blank and was to be configured in the code behind. The solution came from this post in the MS SL forum. For some reason, in all other browsers except IE, the initParams value must contain a value. I followed Eric’s solution and my problem was solved.