--- a/browser/components/preferences/in-content-new/subdialogs.js
+++ b/browser/components/preferences/in-content-new/subdialogs.js
@@ -323,16 +323,17 @@ SubDialog.prototype = {
     this._overlay.dispatchEvent(new CustomEvent("dialogopen", {
       bubbles: true,
       detail: { dialog: this },
     }));
     this._overlay.style.visibility = "visible";
     this._overlay.style.opacity = ""; // XXX: focus hack continued from _onContentLoaded
 
     if (this._box.getAttribute("resizable") == "true") {
+      this._onResize = this._onResize.bind(this);
       this._resizeObserver = new MutationObserver(this._onResize);
       this._resizeObserver.observe(this._box, {attributes: true});
     }
 
     this._trapFocus();
 
     // Search within main document and highlight matched keyword.
     gSearchResultsPane.searchWithinNode(this._titleElement, gSearchResultsPane.query);

--- a/browser/components/preferences/in-content/subdialogs.js
+++ b/browser/components/preferences/in-content/subdialogs.js
@@ -323,16 +323,17 @@ SubDialog.prototype = {
     this._overlay.dispatchEvent(new CustomEvent("dialogopen", {
       bubbles: true,
       detail: { dialog: this },
     }));
     this._overlay.style.visibility = "visible";
     this._overlay.style.opacity = ""; // XXX: focus hack continued from _onContentLoaded
 
     if (this._box.getAttribute("resizable") == "true") {
+      this._onResize = this._onResize.bind(this);
       this._resizeObserver = new MutationObserver(this._onResize);
       this._resizeObserver.observe(this._box, {attributes: true});
     }
 
     this._trapFocus();
   },
 
   _onResize(mutations) {