In the interest of being as brief as possible (just ask for details if you want to know more or if you think that it would help), I have started, against my preferences, to start using chromium from Debian instead of Firefox (aka, Iceweasel).
The reason? I have a slow computer (a Core 2 Duo T7250). My main computer. My workhorse.
There are some sites that even with an empty profile and a vanilla Firefox build right from Mozilla Foundation cause Firefox to generate a very heavy CPU load. I briefly reported this via Twitter to one (but not the only!) of the sites in question and to Firefox Site Compatibility.
What do I see? Many CSS and redraw events (thanks to Firefox's profiling tools). In fact, as many events that Firefox alone is able to completely take over 1 (of the 2) cores that I have, with just that single tab open.
I don't see those problems with Chromium, unfortunately (or, at least, they are not that perceptible to my computer's CPU).
So, before I go on and formally file a bug report, I would love to confirm if other people see the same problems that I am seeing. Do you also see something similar? Your input is highly welcome!