I was wondering why the little avatar was showing a generic Linux Tux instead of the Fedora symbol on my posts, it seemed strange that it stopped working in F15. Anyway, the User Agent Switcher add-on fixes that; in the Options panel just create a New User Agent, name it and change the User Agent field from this
Mozilla/5.0 (X11; Linux x86_64; rv:2.0.1)...
Mozilla/5.0 (X11; Fedora Linux x86_64; rv:2.0.1)...
then select to use that new User Agent in Firefox's Tools menu. That'll have to do until the Fedora packagers update the Firefox package.