Issue: Missing Plugin for Contacts Pop-up?

I use the contacts pop-up on other forums too, but thing is: only on AF when there is at least one person online it displays a "missing plugin" bar.

I'm using Firefox 3.6.x and when i click the "install missing plugins" Firefox doesn't come up with anything, not even suggestion for manual install.

Do you know what is that missing plugin? It gets annoying after a time, plus i like to have plugins so i would really install it if i knew what is it.