The full-stop disabling of legacy extensions will absolutely wreck my current workflow (Tab Groups, Vertical Tabs, Tile Tabs, etc.), to the point where I may need to explore alternatives. I don't like that thought, but I also don't see Mozilla giving power users like me much choice.
I'm kind of in the same boat as you. Vimperator is how is how I use my browser. Vimperator is pretty amazing in terms of functionality, but is also a good illustration why the WebEx migration is needed.
Vimperator can reload every tab every 10 seconds, create a temp buffer and open it in vim for textareas, control other extensions, modify headers or content of the HTML, control the chrome, and so on. From a developer or security standpoint having a plugin that can do this is insane.
That being said, I don't know what I'm going to do when the legacy stuff is fully deprecated. The plugins I use have become so involved in my workflow that I can't really picture Foxfire without them.
This way 99% of the users will never use unsafe addons, the 99% who won't even miss them. But power user who understand and want to, let them ultimately have the freedom to do "stupid" things (what you consider as stupid).
Whether Firefox add-ons should be restricted to what Google Chrome supports is another issue.
It's not, though. See: https://discourse.mozilla.org/t/support-ublock-origin/6746/4...
Many of the major performance improvements in Chrome were possible because the extensions were decoupled from the browser itself. If Firefox ever wants to improve its performance, it can not keep running XUL extensions anymore.
Where do you see the security implications, i.e. what is the threat model here?
0) Accept the loss, grieve, then move on to 57+. 1) Use ESR until 57+ becomes ESR someday. 2) Fork Firefox, rebasing your patch onto upstream. 3) Write a WebExtension, requesting APIs if necessary. 4) Stop updating Firefox, surf the web unsafely.
As a power user, each of these is within your theoretical capabilities. I cannot predict which is the appropriate tradeoff, though.