Trying to update to Iceweasel 18 produces this message:
Some packages could not be installed. This may mean that you have requested an impossible situation or if you are using the unstable distribution that some required packages have not yet been created or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies: iceweasel: Depends: xulrunner-18.0 (>= 18.0-1) but it is not going to be installed E: Unable to correct problems, you have held broken packages.It seems Iceweasel 18 depends on xulrunner-18.0 which depends on Libnss3 2.3.14, which isn't in Testing or Experimental.
The problem is described here and here.