diff --git a/web/app.js b/web/app.js
index ed900ce309435..8850a45e765b5 100644
--- a/web/app.js
+++ b/web/app.js
@@ -201,6 +201,8 @@ const PDFViewerApplication = {
   _isScrolling: false,
   editorUndoBar: null,
   _printPermissionPromise: null,
+  // Only used to keep the preloaded icons alive, see `load`.
+  _preloadedIcons: null,
 
   // Called once when the document is loaded.
   async initialize(appConfig) {
@@ -1911,6 +1913,63 @@ const PDFViewerApplication = {
           this.pdfLayerViewer.render({ optionalContentConfig, pdfDocument });
         });
       }
+      if (
+        typeof PDFJSDev !== "undefined" &&
+        PDFJSDev.test("MOZCENTRAL && !GECKOVIEW") &&
+        !this._preloadedIcons
+      ) {
+        // The CSS only fetches an icon once its element is rendered, hence the
+        // UI hidden on load would first be displayed without its icons. Warm
+        // the image cache up, once the first page is rendered in order to not
+        // delay it, with the icons that aren't already preloaded in
+        // viewer.html (where only the ones visible on load are).
+        // NOTE: When modifying the icons used in the toolbars or in the views
+        // manager, please keep this list and the viewer.html one in sync.
+        this._preloadedIcons = [];
+        window.requestIdleCallback(() => {
+          const path = AppOptions.get("imageResourcesPath");
+          for (const name of [
+            // Secondary toolbar, which also contains the print and download
+            // buttons when the window is narrow (see the `hiddenMediumView`
+            // class in viewer.css).
+            "toolbarButton-print.svg",
+            "toolbarButton-download.svg",
+            "toolbarButton-presentationMode.svg",
+            "toolbarButton-bookmark.svg",
+            "secondaryToolbarButton-firstPage.svg",
+            "secondaryToolbarButton-lastPage.svg",
+            "secondaryToolbarButton-rotateCw.svg",
+            "secondaryToolbarButton-rotateCcw.svg",
+            "secondaryToolbarButton-selectTool.svg",
+            "secondaryToolbarButton-handTool.svg",
+            "secondaryToolbarButton-scrollPage.svg",
+            "secondaryToolbarButton-scrollVertical.svg",
+            "secondaryToolbarButton-scrollHorizontal.svg",
+            "secondaryToolbarButton-scrollWrapped.svg",
+            "secondaryToolbarButton-spreadNone.svg",
+            "secondaryToolbarButton-spreadOdd.svg",
+            "secondaryToolbarButton-spreadEven.svg",
+            "secondaryToolbarButton-documentProperties.svg",
+            // Views manager (see views_manager.css) and its menu.
+            "pages_viewButton.svg",
+            "pages_viewArrow.svg",
+            "checkmark.svg",
+            "toolbarButton-viewOutline.svg",
+            "toolbarButton-viewAttachments.svg",
+            "toolbarButton-viewLayers.svg",
+            "pages_closeButton.svg",
+            "altText_spinner.svg",
+            "altText_done.svg",
+          ]) {
+            const image = new Image();
+            image.src = path + name;
+            // The images are never read: keeping a reference to them prevents
+            // their entries, which expire when unused, from being evicted from
+            // the image cache before the icons are displayed.
+            this._preloadedIcons.push(image);
+          }
+        });
+      }
     });
 
     this._initializePageLabels(pdfDocument);
diff --git a/web/viewer.css b/web/viewer.css
index 0e0cc3741793c..684484e55b853 100644
--- a/web/viewer.css
+++ b/web/viewer.css
@@ -1867,7 +1867,8 @@ dialog :link {
   }
 }
 
-/* Account for Nova's larger toolbar buttons. */
+/* Account for Nova's larger toolbar buttons. The icon preloads in viewer.html
+   use these breakpoints. */
 /*#if MOZCENTRAL*/
 @media -moz-pref("pdfjs.enableNova") and -moz-pref("browser.nova.enabled") {
   @media (max-width: 880px) {
diff --git a/web/viewer.html b/web/viewer.html
index 012ff1a5d90b7..35a42581b7689 100644
--- a/web/viewer.html
+++ b/web/viewer.html
@@ -60,7 +60,27 @@
     <!--#include viewer-snippet.html-->
     <!--#endif-->
 
+    <!-- In Firefox, preload the icons visible on load, i.e. the main toolbar
+         ones (the media queries match the breakpoints of the Nova toolbar in
+         viewer.css), since the CSS only fetches them once rendered. The icons
+         of the UI hidden on load are fetched later, from
+         PDFViewerApplication.load in app.js: please keep both lists in sync. -->
     <!--#if MOZCENTRAL-->
+    <!--<link rel="preload" as="image" href="resource://pdf.js/web/images/toolbarButton-viewsManagerToggle.svg">-->
+    <!--<link rel="preload" as="image" href="resource://pdf.js/web/images/toolbarButton-pageUp.svg" media="(min-width: 801px)">-->
+    <!--<link rel="preload" as="image" href="resource://pdf.js/web/images/toolbarButton-pageDown.svg" media="(min-width: 801px)">-->
+    <!--<link rel="preload" as="image" href="resource://pdf.js/web/images/toolbarButton-zoomOut.svg">-->
+    <!--<link rel="preload" as="image" href="resource://pdf.js/web/images/toolbarButton-zoomIn.svg">-->
+    <!--<link rel="preload" as="image" href="resource://pdf.js/web/images/comment-editButton.svg">-->
+    <!--<link rel="preload" as="image" href="resource://pdf.js/web/images/toolbarButton-editorSignature.svg">-->
+    <!--<link rel="preload" as="image" href="resource://pdf.js/web/images/toolbarButton-editorHighlight.svg">-->
+    <!--<link rel="preload" as="image" href="resource://pdf.js/web/images/toolbarButton-editorFreeText.svg">-->
+    <!--<link rel="preload" as="image" href="resource://pdf.js/web/images/toolbarButton-editorInk.svg">-->
+    <!--<link rel="preload" as="image" href="resource://pdf.js/web/images/toolbarButton-editorStamp.svg">-->
+    <!--<link rel="preload" as="image" href="resource://pdf.js/web/images/toolbarButton-print.svg" media="(min-width: 881px)">-->
+    <!--<link rel="preload" as="image" href="resource://pdf.js/web/images/toolbarButton-download.svg" media="(min-width: 881px)">-->
+    <!--<link rel="preload" as="image" href="resource://pdf.js/web/images/toolbarButton-secondaryToolbarToggle.svg">-->
+    <!--<link rel="preload" as="image" href="resource://pdf.js/web/images/loading.svg">-->
     <!--<link rel="stylesheet" href="resource://pdf.js/web/viewer.css">-->
     <!--<link rel="localization" href="toolkit/pdfviewer/viewer.ftl"/>-->
     <!--#else-->