continuing firefox woes: wrong USE_LANGUAGE, missing GCC_REQD

[Greg Troxel <gdt%lexort.com@localhost>]

I am about to build the newly-updated firefox.

Looking in mozilla-common.mk, I find this:

  # Some modules written in Rust dislike gnu++17 as of 74.0.
  USE_LANGUAGES+=         c99 c++

I won't understand how this can be correct; I have heard that firefox
requires C++17, and the above declares that C++03 is enough.

Even if there is some issue with 17, shouldn't that say c++14?

Also, there is no GCC_REQD.  NetBSD 8 -- squarely on the supported list
of pkgsrc -- has gcc5, and I have the impression that it is known to
fail.