I see this behavior with Iceweasel 10.0.5 from wheezy as well. I'm up-to-date as of right now, and starting with "iceweasel -safe-mode" with one tab open consumes ~120% CPU (dual core macmini 3,1 with 8GiB RAM).
I started noticing this "recently", which isn't to say it hasn't been happening for a while but I usually have two running in two different X sessions with somewhere around 120 tabs open and it has been pretty unresponsive lately. Any more information that would be useful? -- mike warren -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org