binutils is a build-time dependency at best, firefox runs fine without binutils installed. The dependency has been present since the first ebuild version, as far as I can tell from CVS.
The deps are currently under review in fx/tb-24.0 which is gonna be stablized very shortly. We should have everything fixed up by then.
Ebuilds have been updated, they should appear in the tree in next few hours.
Hi, did you forget to push the changes? http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/www-client/firefox/
Or perhaps I misunderstood that the stable ebuild would have been updated as well.