I rely mainly on one add-on: Vimperator. It's a big big productivity enhancer and it's the main reason for sticking with Firefox for so long. Even in a period when Firefox was slower and buggier than the alternatives.

It's sad they will deprecate the old extension interface and make Vimperator a no-no. I understand the reasons behind WebExtensions, specially those related to security, and reckon the benefits of the multiprocess paradigm, but the flexibility of the old extensions API was a big advantage in my pov.

No, Vimperator won't be ported (I wish it could), because WebExtensions is a much more limited API. And no, Vimium and VimFx doesn't come even close to it. They are not alternatives.