Created attachment 216733 [details]
Tail end of build log with OPTIMIZED_CFLAGS off
Turning OPTIMIZED_CFLAGS option off causes compile to break as shown in attached build log, seeming to indicate that libepoll-shim was not linked into the build. Perhaps optimization removes all the calls to the library.
Regressed by https://bugzilla.mozilla.org/show_bug.cgi?id=1619882 which added dependency on epoll and made VAAPI videos transparent. Both MOZ_ENABLE_WAYLAND and MOZ_X11_EGL use DMAbuf for VAAPI. During Firefox 75-77 VAAPI did work fine on Wayland (see bug 244793) but X11 support was added in Firefox 80.
I'm glad to know I wasn't imagining things! I honestly don't know why I ever turned OPTIMIZED_CFLAGS off, but I guess I'll leave it on in the future.