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

• Open Iceweasel and disable this option:

• 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.

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:

• 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.