As far as I understand, Mozilla's main point is how these technologies
prevent Firefox's core from easily evolving, especially performance-wise.
An exception was made for Flash but it is easier to maintain NPAPI working
for Flash than for any plugin. That said, I had rather see Flash
It is not like NPAPI, XUL and XPCOM are dropped with no reason.
NPAPI "plugins are a source of performance problems, crashes, and security
incidents":
https://blog.mozilla.org/futurereleases/2015/10/08/npapi-plugins-in-firefox/
WebExtensions' API does not expose Firefox's internals as much
