This would work reasonably well. It is still circumventable by "attackers" (eg by replacing or patching firefox.exe, modifying /etc/hosts, etc), but the bar is

This would work reasonably well. It is still circumventable by "attackers" (eg by replacing or patching firefox.exe, modifying /etc/hosts, etc), but the bar is higher. On the other hand, it also requires Mozilla running and maintaining a service, which is a pretty resource intensive solution for such a minor feature. Also some users do not like their browsers phoning home for any reason whatsoever, so they would want a way to disable the feature. Of course, once there's a disable switch, attackers can flip it as easily as users.