Got Yahoo! Mail Beta recently.
And one thing I happened to notice was that on Internet Explorer, an ad would come up on the right hand side. A 'skyscraper' ad. But on Mozilla Firefox, no ad on the right side. Just a blank area. The inbox pane and the reading pane did not expand to fill up the space though. Maybe Firefox Adblock plugin is the reason.
By the way, get your name in the source code of Firefox by getting a friend to download it. How? Details @ Firefox day site.