@Moonchild @ On why Iceweasel-UXP doesn't include WebExtensions.:
It exposes one of the multiple hacks Mozilla has used to make WE work in the first place, and maybe even a fundamental flaw in how WebExtensions work, overall (I haven't had time to fully analyze the problem yet). Since it's at the very root of the implementation of WEs and how they interact with web content, I also don't see a straightforward way around that problem.

The issue is that in an effort to hide the fact that a certain extension is installed by randomizing access URLs, that randomized nature creates a unique fingerprint for content to read. So you're in effect going from "this user has extension X installed, it could be user Y or anyone else we've seen with this extension" to "this is user Y". As you can understand, that's a problem ;) -- since the WE needs to know its own random ID so it can access its resources from page content, that ID can't change once it's been established.

Aside from this privacy issue, our continued experimental support in Basilisk depends on how much of a problem our limited implementation (restricting WEs to only content-related functions and not exposing many browser-specific APIs, because that is what we have our other extension types for) poses. Time will tell.