Thanks for beginning a cookie manager for Fx 57+. I know it's brand new & takes a lot of work. Just trying to give some useful suggestions.

1) Re: Taurean75's comment: https://addons.mozilla.org/en-US/firefox/addon/a-cookie-manager/reviews/896808/ - -
yes, I think they meant as in the link you posted: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/user_interface/Popups.
That's the way many addons provide menus, where options may require frequent changing.

2) If I understand, why have any addon open in a NEW TAB, instead of using a toolbar icon? Not a requirement for Fx 57 / webextensions? Or just the fastest way to get Cookie Manager up & running?

Most Fx 57 ready addons I use allow adding icons with drop menus to toolbar. Few require using about:addons to *change* (most) options - especially for cookies.

3) Suggestion: post a few more screenshots on MAO w/ some explanations. Probably entice more users to try it. I see Cookie Monster's dev has pulled it from AMO. Lots of users will be looking for replacement.

Keep up the good work. For ideas, I'd look at features in the more popular cookie managers - that aren't updating to webextensions.

If you want feedback of what's most important to users to implement 1st, maybe setup a support page on http://forums.mozillazine.org, as other addon devs did. One post could be a poll for wish list.

I could tell you some cookie features I've used most - for years, & read others say were important to them, but it'd make this review long(er). Maybe you don't need ideas for now. :) But, feel free to PM me.

Edited review 08/13/2017: correct typos