Vimperator has an issue in it's github referring to e10s and webextensions from 2015. It was commented upon a few times through 2015, more in 2016, and obviously more now. Nobody ever started moving forward. Only 10 days ago, a different issue was opened with regards to porting to an entirely new codebase for vimperator that uses WebExtension. They just now are talking about getting in touch with Mozilla for API requests.
Mind you, a year plus ago, multiple mozilla developers were holding office hours blocked out in the day to explicitly talk to e10s and webextension developers. It got scaled back because nobody ever talked to them and paid developers cannot be paid to do nothing for hours at a time when nobody calls in/emails/etc.
Meanwhile OTHER plugins HAVE taken this seriously, but not a lot of them. Certainly not enough. And yes, that means that you are going to have those plugins lose compatibility. But that's not firefox's fault. Mozilla has done everything they can do to help this transition, including paying people to help, pushing it back multiple times, giving numerous conferences, opening up api requests, and communicating on most every forum available on the internet.
It's on the plugin developers now. That's where your anger should be. Not on the company that is doing, finally, what it said it'd do numerous years ago.
As someone who doesn't follow the nitty-gritty of Firefox development, it's frustrating to only really find out about it when things stop working.
I'm sure they had their reasons for deciding what they did. I still think it's a bad decision.
1) vimperator 2) single process handling multiple tabs (I typically have upwards of 40 tabs open at a time)
Firefox is in the process of removing plugins like 1) so that it makes it possible to remove 2).
Wonderful.