ALT tags in Mozilla

There seems to be some pretty good Mozilla expertise
here, and this may be a FAQ, but: In IE they can set an
option so that the contents of ALT tags shows up in
popup windows when you drag your cursor over images. I
haven't been able to find where or even whether this is
implemented in Mozilla. Evidently it's not? This is a
major usability issue.

Thanks.

--
Lynn David Newton
Phoenix, AZ