--- a/browser/components/preferences/in-content-new/findInPage.js
+++ b/browser/components/preferences/in-content-new/findInPage.js
@@ -14,31 +14,32 @@ var gSearchResultsPane = {
     if (this.inited) {
       return;
     }
     this.inited = true;
     this.searchInput = document.getElementById("searchInput");
     this.searchInput.hidden = !Services.prefs.getBoolPref("browser.preferences.search");
     if (!this.searchInput.hidden) {
       this.searchInput.addEventListener("command", this);
-      window.addEventListener("load", () => {
+      window.addEventListener("DOMContentLoaded", () => {
         this.searchInput.focus();
-        this.initializeCategories();
       });
+      // Initialize other panes in an idle callback.
+      window.requestIdleCallback(() => this.initializeCategories());
     }
     let strings = this.strings;
     this.searchInput.placeholder = AppConstants.platform == "win" ?
       strings.getString("searchInput.labelWin") :
       strings.getString("searchInput.labelUnix");
   },
 
   handleEvent(event) {
-    if (event.type === "command") {
-      this.searchFunction(event);
-    }
+    // Ensure categories are initialized if idle callback didn't run sooo enough.
+    this.initializeCategories();
+    this.searchFunction(event);
   },
 
   /**
    * Check that the passed string matches the filter arguments.
    *
    * @param String str
    *    to search for filter words in.
    * @param String filter