The really galling thing is they lied right to our damn faces about how "the new faster systems breaks the old add-on system" while they were still using XUL behind the scenes. In fact an HN reader compiled versions of Firefox 57 and above with user-installed XUL enabled and they work perfectly well.
Nothing is really stopping people from keeping those extensions compatible. Unsupported doesn't mean impossible - it just means you need to do some tweaks: https://webextensions-experiments.readthedocs.io
I'm not understanding how that relates to the fact that the only thing stopping users from installing their XUL extensions on FF57 was a software switch they weren't allowed to touch without editing and compiling from source.
No editing or compilation needed. You just needed to disable signature checks (xpinstall.signatures.required) and enable extension experiments (extensions.experiments.enabled) in a developer edition version (or nightly) version of the browser.
