Be aware that there's currently an issue with Windows Presentation Foundation 3.5.30729.1 in Firefox browsers which may cause the below dialog to appear when launching certain Web sites in Firefox (in my case I'm running Firefox 3.5.3 and was accessing this site - http://msdn.microsoft.com/en-au/library/ms442469%28office.14,lightweight%29.aspx - when I received the error dialog).

A recent article on Computerworld explains the current issue: -"Mozilla blocks Microsoft's sneaky Firefox plug-in":  http://www.computerworld.com/s/article/9139518/Mozilla_blocks_Microsoft_s_sneaky_Firefox_plug_in?taxonomyId=125