MB 87: Disable the default browser box on Windows and Linux

Windows and Linux will be distributed only as portable apps at the
beginning, so they should not be settable as default browsers.
We will need to improve the logic once we decide to ship system-wide
installers, too.

browser/components/preferences/main.js

@@ -1744,7 +1744,9 @@ var gMainPane = {
       let defaultBrowserBox = document.getElementById("defaultBrowserBox");
       let isInFlatpak = gGIOService?.isRunningUnderFlatpak;
       // Flatpak does not support setting nor detection of default browser
-      if (!shellSvc || isInFlatpak) {
+      // privacy-browser#87: Let's keep the default browser box only on macOS
+      // for now.
+      if (!shellSvc || isInFlatpak || Services.appinfo.OS !== "Darwin") {
         defaultBrowserBox.hidden = true;
         return;
       }