diff --git a/devtools/client/debugger/src/client/firefox.js b/devtools/client/debugger/src/client/firefox.js
index fb0734152604..d0868a9eb3bc 100644
--- a/devtools/client/debugger/src/client/firefox.js
+++ b/devtools/client/debugger/src/client/firefox.js
@@ -9,13 +9,15 @@ import { setupEvents, clientEvents } from "./firefox/events";
 import { features, prefs } from "../utils/prefs";
 
 let actions;
+let targetList;
 
 export async function onConnect(
   connection: any,
   _actions: Object
 ): Promise<void> {
-  const { devToolsClient, targetList } = connection;
+  const { devToolsClient, targetList: _targetList } = connection;
   actions = _actions;
+  targetList = _targetList;
 
   setupCommands({ devToolsClient, targetList });
   setupEvents({ actions, devToolsClient });
@@ -40,6 +42,19 @@ async function onTargetAvailable({
   targetFront,
   isTargetSwitching,
 }): Promise<void> {
+  const isBrowserToolbox = targetList.targetFront.isParentProcess;
+  const isNonTopLevelFrameTarget =
+    !targetFront.isTopLevel &&
+    targetFront.targetType === targetList.TYPES.FRAME;
+
+  if (isBrowserToolbox && isNonTopLevelFrameTarget) {
+    // In the BrowserToolbox, non-top-level frame targets are already
+    // debugged via content-process targets.
+    // Do not attach the thread here, as it was already done by the
+    // corresponding content-process target.
+    return;
+  }
+
   if (!targetFront.isTopLevel) {
     await actions.addTarget(targetFront);
     return;
diff --git a/devtools/client/framework/toolbox.js b/devtools/client/framework/toolbox.js
index 166ae4825795..a2de31397a8c 100644
--- a/devtools/client/framework/toolbox.js
+++ b/devtools/client/framework/toolbox.js
@@ -738,7 +738,8 @@ Toolbox.prototype = {
 
     const isBrowserToolbox = this.targetList.targetFront.isParentProcess;
     const isNonTopLevelFrameTarget =
-      !targetFront.isTopLevel && targetFront.type === TargetList.TYPES.FRAME;
+      !targetFront.isTopLevel &&
+      targetFront.targetType === TargetList.TYPES.FRAME;
 
     if (isBrowserToolbox && isNonTopLevelFrameTarget) {
       // In the BrowserToolbox, non-top-level frame targets are already
@@ -766,7 +767,7 @@ Toolbox.prototype = {
   _attachAndResumeThread: async function(target) {
     if (target.threadFront) {
       // if threadFront already exists, the thread is already attached.
-      if (target.type !== TargetList.TYPES.SERVICE_WORKER) {
+      if (target.targetType === TargetList.TYPES.SERVICE_WORKER) {
         // This can legitimately happen for service workers. See Bug 1655439.
         console.warn(
           "Attaching to an already attached thread for a service worker target"
@@ -778,7 +779,7 @@ Toolbox.prototype = {
 
       // This should not happen for non-server-worker targets, throw otherwise.
       throw new Error(
-        `Attaching to an already attached thread for a target of type "${target.type}"`
+        `Attaching to an already attached thread for a target of type "${target.targetType}"`
       );
     }