I don't think it's IE "by itself" that uses up less power; rather, I think it's Flash for IE that uses system resources better than the 'Netscape plug-in" version (Adobe Flash comes in two formats, in Windows: ActiveX control, for IE; Netscape plug-in, for all other browsers), as 'Firefox+AdBlock' demonstrates: all Flash adds are removed in Firefox, but stuff like YouTube videos are not (since they are part of the site and not blacklisted by AdBlock).
Another test that could be done, is Firefox+Adblock+NoScript. If you try THAT combination, you'll probably reduce power use by quite a lot (if you don't spend all your time watching YouTube, I mean; if you do, that won't save squat).
Imagine, only opening Flash videos you really want to watch...
What do we learn from that? Well, Flash in ActiveX is more resources-efficient than a Netscape plugin. But wait, ain't ActiveX an entry door for virii? Ooooh...