https://blog.mozilla.org/futurereleases/2015/10/08/npapi-plugins-in-firefox/
Mozilla, the creators of Firefox, is working closely with affected publishers to make the NPAPI Plugin transition as painless as possible.
No, thank you.
That article, which is from 2015 btw, explain it well. NPAPI caused crashes, and a lot of security issues, that's why they were removed.
There are multiple causes to its demise.
The big one was security (or lack thereof) as attackers would abuse plug-ins through NPAPI. I remember a time when every month had new 0-days exploiting a vulnerability in Flash.
The second one in my opinion, is the desire to standardize features in the browser. For example, reading DRM-protected content required Silverlight, which wasn't supported on Linux. Most interactive games and some websites required Flash which had terrible performance issues. So it felt natural to provide these features directly in the browser without lock-in.
Which leads to your second question: I don't think we will ever see the return to NPAPI or something similar. The browser ecosystem is vibrant and the W3C is keen to standardize newly needed features. The first example that comes to mind is WebAuthn: it has been integrated directly in the browsers when 10 years ago it would have been supported through NPAPI.
If I remember correctly, the issue was they were moving to sandboxing and NPAPI plugins had a lot of issues with security at the time. There was a new flash vulnerability almost every week, and usually it meant getting a lot of control over the browser.
i kind of miss it, because there was an tribute game called aot tribute game it was very fun, i used to play it alot back in 2015, and i remember it only ran on firefox after chrome booted out NPAPI
This is almost ten years old, and NPAPI plugins have been desupported by pretty much everything except Pale Moon, which forked from Firefox so long ago that it also still supports XUL.