Thanks to bernat on #debian @ irc.debian.org for helping me track down this bug and devise a workaround.
When working on my wife’s netbook, I noticed that when idling on Facebook in iceweasel 24.3.0esr-1, the process was taking far too much CPU. I then retested on a wheezy system with the release iceweasel from mozilla.d.n, which at that time was 26, and I later upgraded to 27. Same problem there, too, on both versions. In fact, it seems the slowdown was amplified by the fact that I was running iceweasel in vnc4server, not the worlds most efficient X implementation.
Even with all these versions tested, I have yet to file a Debian bug, as I will need some time on a system where the slowdown is noticeable and I’m using a current Debian version. But I wanted to post now to give props to bernat for his help. If you think you have this issue, go read his article linked above, which contains the workaround.