--- a/devtools/client/framework/toolbox-process-window.js
+++ b/devtools/client/framework/toolbox-process-window.js
@@ -65,16 +65,17 @@ function setPrefDefaults() {
   Services.prefs.setBoolPref("devtools.inspector.showUserAgentStyles", true);
   Services.prefs.setBoolPref("devtools.performance.ui.show-platform-data", true);
   Services.prefs.setBoolPref("devtools.inspector.showAllAnonymousContent", true);
   Services.prefs.setBoolPref("browser.dom.window.dump.enabled", true);
   Services.prefs.setBoolPref("devtools.command-button-noautohide.enabled", true);
   Services.prefs.setBoolPref("devtools.scratchpad.enabled", true);
   // Bug 1225160 - Using source maps with browser debugging can lead to a crash
   Services.prefs.setBoolPref("devtools.debugger.source-maps-enabled", false);
+  Services.prefs.setBoolPref("devtools.debugger.new-debugger-frontend", false);
 }
 
 window.addEventListener("load", function() {
   let cmdClose = document.getElementById("toolbox-cmd-close");
   cmdClose.addEventListener("command", onCloseCommand);
   setPrefDefaults();
   connect().catch(e => {
     let errorMessageContainer = document.getElementById("error-message-container");