The support for extensions relying on the older (and problematic, because
tightly coupled with Firefox's core) XUL and XPCOM APIs have been disabled
since Firefox 57, in favor of WebExtensions. After a rather long period of
transition. Indeed, the WebExtension API has been considered stable since
Firefox 48;
https://blog.mozilla.org/addons/2016/04/29/webextensions-in-firefox-48/
Abrowser 58 being a derivative of Firefox 58, it only supports WebExtensions.
GNU IceCat is a derivative of Firefox LTS. As a consequence, it is still at
version 52 and supports older extensions.
The changes that Firefox 57 brought were discussed on this forum:
https://trisquel.info/forum/quantum-kicks-ass
