The high level API s**t is what's killing the (Firefox) platform

I don't think a rant from last year has much value. And it's worth noting that DownThemAll is being ported to WebExtensions after all:

http://www.downthemall.net/progress/

The WebExtensions API isn't frozen for all time. I'm sure if the DownThemAll developers get involved with specifying and designing the APIs they need then Mozilla will consider and add that functionality in future Firefox releases.