Supporting legacy addons is not the real problem here
The actual problem is that the new WebExtensions API can only do a fraction of the things the old API did. Simply put, some old extensions cannot be rewritten, because the new API simply won't let you modify the browser to the same extent. Mozilla did put in some effort to include additional APIs, but at the end of the day, most of these issues were met with a simple shrug.
Considering its addons were pretty much the only edge Firefox had left compared to Chrome, that's the real tragedy right there.
For me, after years of putting up with one idiotic design decision after another, this was the last straw. I am now using Pale Moon, where 90% of my addons work, and will continue to work for the foreseeable future. They might even add WebExtensions in later on. Goodbye and good riddance, Firefox.
Biting the hand that feeds IT
