I&#x27;m a Mozilla dev who has spent way too much time working on NPAPI over the past few years. The major problem with NPAPI IMHO is that everything is reentrant. Calls to plugins can call back into the browser, which then can call back into the plugin, ad nauseum. This is all happening on the main thread of the browser, BTW. OK, now throw out-of-process into the mix, so your IPC mechanism has to support all of this insanity, plus still integrate properly with the platform&#x27;s event queue (Windows I&#x27;m looking at you). It&#x27;s a stability nightmare.