On one of my machines (debian squeeze), I have just upgraded the standard version of Iceweasel provided with debian squeeze stable (3.5.16). I am now using the version available with the backports repository (18.0.1). I have upgraded Icedove, too.
When Iceweasel or Icedove are called/run on the machine where they are installed, everything is fine. However, I have trouble using the new versions via ssh and through X forwarding. I am fairly sure it's not a basic issue with the X forwarding configuration because other applications (like gimp or an earlier version of Iceweasel) work well.
Here's a screenshot of Iceweasel 3.5.16 (from a machine called "box"), working fine:
This is the blank window I get with the newer version of Iceweasel (18.0.1) or Icedove (10) through ssh/X forwarding on the same notebook, just called from a different machine ("boom") for test reasons:
One more hint: When I called the same new version of Iceweasel from an older debian istallation (lenny), I got a display in the window, but it crashed a lot (e.g. segmentation fault for some sizing issue with dialog boxed like Google's input bar).
Since the exact same problem exists with both Iceweasel and Icedove, I am led to thinking that the newer versions of mozilla software are either buggy when confronted with X forwarding or require a configuration of X forwarding that I am not aware of.
... probably the latter. Things work when I access Iceweasel from one notebook (i386/debian squeeze), and the problems occur when I use another notebook (ppc/debian squeeze). Maybe I need some good hints in which config files I should look for differences...