You actually need to click Gmail's "Show Original" link in the message to view the emails Source Code. Then look for the link that looks like this:
http://ideas.live.com/AcceptInvitation.aspx?[bunchofletters&numbers]
Then you'll need to sign in, and then when you get to the next download button, it won't work in firefox either, so you'll need to view the source of the page again to find the download link.
Microsoft sure is making it difficult for Gmail and Firefox users.
