Chrome isolates the content of each domain’s tabs in a separate process – which lets the user manage each of them with the operating system’s native process management tools. Firefox does not – so when it starts hogging 100% CPU, users are clueless.
Among the usual suspects is Flash, but Flash is innocent on my workstation’s Iceweasel : I entirely removed any Flash interpreter from it and I am now clean from this filth.
And that’s it – Iceweasel’s CPU usage is back to near zero, where it belongs. In my case, among the ocean of open tabs in on my virtual desktop’s many open windows, the culprit was this page to whose incompetence I grant some Pagerank as a token of appreciation for having led me to discover a solution to this problem !