I think that it ought to be configurable. If you do not specify a DNS proxy in Firefox, then the operating system's DNS should be used. If it does have its own service activated (which it shouldn't; this should be a part of the operating system instead and not part of Firefox), then it makes sense it won't pay attention to the hosts file (although then they would have to add their own implementation of such thing, perhaps with there own hosts file, which might just be a symlink to the operating system hosts file if that is what the user wants).
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I think that it ought to be configurable. If you do not specify a DNS proxy in Firefox, then the operating system's DNS should be used. If it does have its own service activated (which it shouldn't; this should be a part of the operating system instead and not part of Firefox), then it makes sense it won't pay attention to the hosts file (although then they would have to add their own implementation of such thing, perhaps with there own hosts file, which might just be a symlink to the operating system hosts file if that is what the user wants).