4 of the top 10 most frequent causes of jank in the browser are due to NPAPI plugins.
To be fair, judging from the call stacks and the method names all but one are hangs within your IPC mechanism. You would need to see the remote process call stacks as well to figure out the exact cause.
Qualitatively speaking, I can tell you from first-hand experience that NPAPI is a really shitty API to work with, is almost 20 years old, and is very challenging to improve while preserving backward compatibility.
Oh, I believe you. I had the questionable pleasure to work with it as well. The question is not whether or not you should get rid of it. You definitely should. But the problem is you aren't. You ruin compatibility to world top 100 sites without any immediate benefit. You will still have to keep this huge blob of code around until at least Shumway is done or Flash is being obsolete and EME is supported on all major platforms. But still for some inexplicable reason you feel the need to just mess with it now.
FWIW, I was told a few weeks ago that NPAPI is roughly 85,000 lines of code.
That sounds about right. I looked over it to get an idea of how hard keeping the seperate x64 version around would be.
In the case of Silverlight, we consider its main use case to be video, particularly Netflix. Since HTML5 video will be able to handle this, the case for supporting Sliverlight in Win64 is much weaker. So weak, in fact, that NPAPI on Win64 is restricted[3] to running Flash.
Except that you don't support it. Sure, Netflix could use a DRM free stream to display video, but I hope it is obvious to you why this isn't going to happen. There are open source ideals and there are corporate and legal realities. And you can bet your ass that those license agreements Netflix, or any other mainstream VOD provider for that matter, have with content providers include clauses that the content has to be protected. If Firefox had a working EME implementation on x64, you would be absolutely right. I could just be a pain in the ass over at Netflix and pester them to release the HTML5 version for Firefox. But I can't, because EME is not supported on Windows x64, OS X, Windows XP and Linux the last time I checked.
Win64 Firefox is in beta and is incomplete; it has not been released yet for a reason.
That reason is not a broken Netflix though. The only reason that has been communicated for why 41 isn't going to be the release version is the lack of proper sandboxing of Flash. If there are different information, maybe communicate them? For example in form of a definitive statement in Bugtraq that a non-functional Netflix is considered a blocker for any Win64 release.