--- a/devtools/client/responsive.html/browser/tunnel.js
+++ b/devtools/client/responsive.html/browser/tunnel.js
@@ -550,16 +550,25 @@ MessageManagerTunnel.prototype = {
       this.destroy();
     }
     if (subject == this.outerParentMM) {
       debug("Outer messageManager has closed");
       this.destroy();
     }
   },
 
+  /**
+   * Expose the inner frame's value for `processMessageManager`.  This is done mainly to
+   * allow Browser Content Toolbox (which needs to find a tab's process) to work for RDM
+   * tabs.  (The property is quite rarely used in general.)
+   */
+  get processMessageManager() {
+    return this.innerParentMM.processMessageManager;
+  },
+
   loadFrameScript(url, ...args) {
     debug(`Calling loadFrameScript for ${url}`);
 
     if (!this.OUTER_TO_INNER_FRAME_SCRIPTS.includes(url)) {
       debug(`Should load ${url} into inner?`);
       this.outerParentMM.loadFrameScript(url, ...args);
       return;
     }