Port to WebExtensions?

[PaulBags]

Currently add-on is legacy, and will stop working from FF57+

Not really pushing, because so many add-ons won't survive I won't be going to 57. But it's not mentioned, and it's not mentioned in the add-on store - might as well create the issue.

[blackwind]

I responded to a review a few months back with:

Impossible, I'm afraid. If the API situation changes, I'll do what I can at that point; until then, you'll need to migrate to Nightly or Waterfox if R+ is important to you.

Since then, nothing has changed, and knowing Mozilla, it never will.

[KrasnayaPloshchad]

Since FoxyTab and FoxyImage released at addons.mozilla.org, I feel no necessary to wait for new version of R+.