RFP: OS locale & HTTP Accept-Language header

https://bugzilla.mozilla.org/show_bug.cgi?id=1459089 . When/if it gets uplifted to 61, I'll update.

user.js

@@ -1576,6 +1576,7 @@ user_pref("privacy.firstparty.isolate.restrict_opener_access", true);
       Modifier events suppressed are SHIFT and both ALT keys. Chrome is not affected.
       FF60: Fix keydown/keyup events (1438795)
  ** 1337157 - disable WebGL debug renderer info (see 4613) (FF60+)
+ ** 1459089 - disable OS locale in HTTP Accept-Language headers (FF62+)
 ***/
 user_pref("_user.js.parrot", "4500 syntax error: the parrot's popped 'is clogs");
 /* 4501: enable privacy.resistFingerprinting (FF41+)