I'm not sure it's new, but I just noticed that Firefox now displays the Live Bookmarks icon when I view my Gmail inbox. The feature itself is not new - Gmail has had an Atom for a few months - but the link is new (Live Bookmarks are "discovered" through a <link> tag in the html header).
The feed has the list of unread messages in the inbox. Firefox displays only the subjects, but proper feed readers (or feed reader extensions in Firefox) can show the sender's information, as well as a snippet.
UPDATE: If you look for Gmail invites, see my other post.