Comment by the_why_of_y

The argument against NaCL was that it was the browser API, PPAPI, was poorly documented and exposing implementation details of Blink/Chromium and thus very difficult to implement in a non-Chromium browser, so it's no surprise that Mozilla, Apple, and Opera were unenthused.

https://bugzilla.mozilla.org/show_bug.cgi?id=729481#c83