Unfortunately not many seem to remember that brief period when the old add-on APIs were being ported and made to work with a multi-process architecture, as part of the effort known as Electrolysis [1], with appreciable results, before being completely killed in favor of WebExtensions. I'm sure there were significant technical challenges, but let's not use these to hide the political reasons behind such a drastic move.
Electrolysis was an easier thing to handle, and some APIs would still have been lost.
Fission (process-per-origin) came later and would have broken many, many more.
So people would have still been pissed off at Mozilla (the difference between levels of breakage would have been lost), and Mozilla would still be on the hook for supporting a barely supportable API. Without the resources that would require.
[1] https://wiki.mozilla.org/Electrolysis