[KwanEsq]

This isn't actually (entirely) true.

One of the core technologies of the old extension model, XBL, has been completely ripped out. XBL-defined elements and XBL overlays were how a lot of the old UI customisations functioned, so even if you enable loading the older code it just flat won't work. Obviously custom JavaScript, as in the example you linked, can still do whatever when loaded without restrictions, but then you're at the mercy of changing internals, instead of the supported stable WebExtensions API.

Additionally a lot of XUL in the chrome UI has been removed, replaced with HTML equivalents or custom elements (a descendant of XBL), and the root documents that make up the browser chrome have transitioned from being .xul XUL documents with some HTML elements in to .xhtml HTML documents with some XUL elements in.

[tn123]

As I said elsewhere, I reviewed thousands of addons for addons.mozilla.org back in the day. The number of extensions which used XBL directly were just a handful, and usually by the most capable developers. I have no doubt these developers would have adopted to XBL going away, especially as WebComponents is basically a replacement for most of it.

That some of XUL has gone or was moved to HTML doesn't really matter much either. People could have adopted to this as well. Adopting to ongoing changes in Firefox was the bread-and-butter of extension development before WebExtensions (or the add-on SDK/jetpack), a necessary cost of doing business with the reward of very powerful things you could then do in and to the browser. With JetPack and the Addon SDK mozilla tried to provide stable APIs with some success. Adding WebExtensions to the mix would have helped, just to make it easier for Chrome extensions devs to port their stuff. People not in need of anything besides these stable APIs could use these and be done. People who needed more powerful capabilities would have went the other route and tracked Firefox changes, just like they used to for over a decade already anyway.

There were tons of major breaking changes over the years (Firefox 3.5 to 4 was particularly "bad" IIRC), and while those caused some causalities over the years with abandoned extensions not getting updated (tho most popular add-ons affected got new maintainers or were forked), most extensions kept going, adapting as needed.

I am pretty sure that I could pick up my "legacy" DownThemAll! code now and today, spend a month adopting it, and have a working version for Firefox 94+. A month may sound like a lot for some hobby project, but in an alternate reality where "legacy" add-ons still would be supported, I would have spent that month over a duration of the last 5 years.

But in this reality, I have no supported way to actually load such a "legacy" extension. I could add such a capability back to a Firefox fork, but then only I would be able to use it, but nobody else would... Unless I released the fork, and really, I don't have the resources to maintain an entire Firefox public fork.