Firefox has been increasingly defeatured over the past year or two. And to make matters worse, the FF developers consider that a feature.
The first big one was requiring add-ons to be signed by Mozilla, putatively to protect users (because Mozilla would inspect the code). That was sort of OK-ish at first, because there was a preference that could be set to turn that off, but they did (as promised) get rid of that option in FF 52. The stated intent was that people could be hurt by rogue extensions coming with instructions about how to turn off the signature enforcement. But it turns out that there is still a saving throw; only add-ons require signature enforcement; other types of addons (such as themes) don't, and the ones that do are listed in a file. Maybe the Mozilla people did that by intent, so that someone who wants to run unsigned extensions badly enough can do so. But yes, this means that you can't run your own extensions in your own browser, unless you submit each new version to Mozilla (not necessarily make it public), or you use the developer version.
(This was never implemented for the long-term support versions; these versions are intended for corporate use, and they know that corporations won't allow their code to be submitted for inspection.)
But the really big change, as of FF57, is to get rid of all of the old extensions altogether in favor of "WebExtensions", which use an API supposedly much more like that of Chrome, to make it easier to port addons between browsers. This strikes me as a highly self-destructive act (why use fake Chrome rather than the real thing?), but that's what they want to do. The problem is, as the OP noted, that none of the classic extensions are WebExtensions, so they're basically destroying their ecosystem overnight.