That is indicating an unresolvable dependency. Are you using the testing or sid repos and what version do you have currently installed?
edit: I should also ask about outside repos. Some, such as Hadret are for Sid and installing things from there or other Sid repos will take you to dead ends like that if you are using testing. If that is the case you will need to wait until the testing repos catch up to the Iceweasel version you are trying to upgrade to. For example the version currently in the Sid repos is 3.5.15-1 which would require an xulrunner equal to or greater than 18.104.22.168, etc. (there are a number of others as well)
arch | kernel26-lqx | kde 4.7.0 & xfce 4.8.1(tiling)
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