--- a/devtools/client/framework/ToolboxProcess.jsm
+++ b/devtools/client/framework/ToolboxProcess.jsm
@@ -272,16 +272,17 @@ BrowserToolboxProcess.prototype = {
     if (!system.constants.MOZILLA_OFFICIAL) {
       args.push("-purgecaches");
     }
 
     this._dbgProcessPromise = Subprocess.call({
       command,
       arguments: args,
       environmentAppend: true,
+      stderr: "stdout",
       environment: {
         // Disable safe mode for the new process in case this was opened via the
         // keyboard shortcut.
         MOZ_DISABLE_SAFE_MODE_KEY: "1",
       },
     }).then(proc => {
       this._dbgProcess = proc;