2699: Navigator API spoof correction

See: https://bugzilla.mozilla.org/show_bug.cgi?id=1383495#c8 - the comment in code is incorrect and hopefully will be fixed. Version is always rounded DOWN

user.js

@@ -1501,7 +1501,7 @@ user_pref("privacy.firstparty.isolate.restrict_opener_access", true);
  ** 1217238 - reduce precision of time exposed by javascript (FF55+)
  ** 1369303 - spoof/disable performance API (see 2410-deprecated, 2411, 2412) (FF56+)
  ** 1333651 - spoof Navigator API (see section 2697) (FF56+)
-      The version number will be rounded to the "nearest" multiple of 10
+      The version number will be rounded down to the nearest multiple of 10
  ** 1369319 - disable device sensor API (see 2512) (FF56+)
  ** 1369357 - disable site specific zoom (see 2515) (FF56+)
  ** 1337161 - disable gamepad API (see 2501) (FF56+)