In December, 2006 Garrett Serack (Fear the Cowboy!) wrote about Detecting CardSpace support, including FireFox. His detection script since made its way onto numerous sites and into relying party software releases.
Unfortunately, this script didn’t detect selectors on Internet Explorer 8 due to changes between IE7 and IE8. Andrew Arnot asked the question Why don’t InfoCards work in IE8? on StackOverflow.com, and then subsequently answered his own question, with help from the IE8 team. Given I’ve referred people to this answer numerous times since, I decided to re-post it here, both for others, and for my own ease of reference.
embed.type = "application/x-informationcard";
Then your relying party will work with IE7, IE8, and Firefox.