The sponsored listings do appear very briefly as the page is loading, but then they disappear. It's probably a design error in FF as the search result description text goes over the right hand ads too.
I'ved used every FF made since Firebird 0.3 and never had the problem with Yahoo ads not showing up. I am now using 1.01 as well and still they are showing up.
I have also used FF on both 98 and XP and they show up on both...
Googlebar 0.9.0.30 Google Preview 0.8 Google PR Status 0.9.3 DOM Inspector 1.0