It wasn't until 2018(?) that they removed the old system for real. And back then they promised to replace all lost functionality with proper designed APIs, to m

It wasn't until 2018(?) that they removed the old system for real. And back then they promised to replace all lost functionality with proper designed APIs, to make all add-ons workable. Never happened beside some low hanging improvements. But the problems with extensions started far earlier. It was a common joke how every bigger update broke many popular extensions, made some of them even impossible. In the early days of Firefox, extensions very growing wild and rich. But over time so many were dying a slow death. At the end of the early days (around Firefox 3 or 4), mostly nothing was left, and Firefox was already significant crippled in ability. The later removal of XUL only gave it the last hit. Today it's basically a handful of popular extensions who keep this feature alive, and most of them are just Adblockers.