Firefox use a lot of xorg resources (much more than other browsers). But I think your problem may be related to hardware acceleration.

- Open Iceweasel and disable this option:

![firefox][1]

- Exit of Iceweasel and look in top if this is closed.
- Run Iceweasel and try again the websites.

Next steps:

- If the problem persist, try with the official version of Firefox.
- If the problem persist with the official version, report to Mozilla.
- If the problem only occur with Iceweasel, report to Debian.

 [1]: https://i.sstatic.net/EDmql.png