JWPlayer support is difficult because there are many JWPlayer variants and old versions out there. It's easier to test and whitelist individual websites to start.
Why bother emulating it, though? If you can recognize it, and recognize that it's not using any unusual settings, you could potentially just replace the whole thing with a native video player.
That's a good question. Video is just one use case for Shumway. This specific use case might be better supported by rewriting Flash <object> tags with an HTML5 video player, but Mozilla dislikes adding hacks to workaround site-specific quirks. The hacks are technical debt carried by Firefox on behalf of websites that don't bother to maintain themselves. Mozilla even removes moz- prefixed DOM APIs and CSS attributes when the standardized, unprefixed name is supported.
