To achieve that we would like devs to provide an .svg outline of their icon instead of a full color icon. This .svg will be colored by us matching the OS (currently a lighter gray on windows). This can also be used to implement the icon highlight state (blue) we currently use in our toolbar-icons. (like bookmarks, download) Using such an icon should be preferred by developers to best integrate their add-ons in Firefox.
A related question... http://stackoverflow.com/questions/37366781/how-to-detect-firefox-theme
Calling out that Screenshots will ship in 54 as a WebExtension. Currently, we our browserAction icon looks janky on dark themes.