why ask about something that hasn't even started?
by the time the webextension api is capable of doing what they're promising, servo might be usable
which is part of the point of moving away from the current extension model: it's hard to work with multiple-processes and rendering engine changes
by the way, ghostery is run by advertising companies. i wouldn't trust it.
and once the WE api _is_ usable, the only thing downthemall would need special is to hook in as the replacement download manager, which is likely as not planned. so i think the developer is overreacting a bit.
i would rather have a browser whose guts i can reach into like firefox works now, but all development trends towards parallelization to get benefits in speed and encapsulation