Bug 1726606: Default browser.display.document_color_use to 1 on MacOS r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D123142

accessible/tests/browser/telemetry/browser_HCM_telemetry.js

@@ -95,11 +95,11 @@ async function setBackgroundColor(color) {
 add_task(async function testInit() {
   const dialogWin = await openColorsDialog();
   const menulistHCM = dialogWin.document.getElementById("useDocumentColors");
-  if (AppConstants.platform == "win" || AppConstants.platform == "macosx") {
+  if (AppConstants.platform == "win") {
     is(
       menulistHCM.value,
       "0",
-      "HCM menulist should be set to only with HCM theme on startup for windows and mac"
+      "HCM menulist should be set to only with HCM theme on startup for windows"
     );
 
     // Verify correct default value
@@ -113,7 +113,7 @@ add_task(async function testInit() {
     is(
       menulistHCM.value,
       "1",
-      "HCM menulist should be set to never on startup for other platforms"
+      "HCM menulist should be set to never on startup for non-windows platforms"
     );
 
     // Verify correct default value

modules/libpref/init/StaticPrefList.yaml

@@ -979,14 +979,19 @@
   value: "#FFFFFF"
   mirror: never
 
-# 0 = always, except in high contrast mode
-# 1 = always
-# 2 = never
+# This preference is a bit confusing because we use the opposite
+# string value in the colors dialog to indicate to users how FF HCM
+# will behave.
+# With resect to document colors, these values mean:
+# 0 = "default" = always, except in high contrast mode
+# 1 = "always"
+# 2 = "never"
 #
-# Default to 0 on windows and mac, 1 elsewhere.
+# On windows, we set this to 0, which means FF HCM will mirror OS HCM.
+# Everywhere else, we set this to 1, disabling FF HCM.
 - name: browser.display.document_color_use
   type: RelaxedAtomicUint32
-#if defined(XP_WIN) || defined(XP_MACOSX)
+#if defined(XP_WIN)
   value: 0
 #else
   value: 1