P4TB-213: Hide Back/Next buttons as appropriate instead of disabling them.

addon/content/tutorial/tutorial.js

@@ -5,17 +5,17 @@ function updateButtons(iframeSrc) {
   let nextButton = document.getElementById("nextButton");
   switch (iframeSrc) {
     case "tutorial-1.html":
-      backButton.disabled = true;
-      nextButton.disabled = false;
+      backButton.hidden = true;
+      nextButton.hidden = false;
       break;
     case "tutorial-2.html":
     case "tutorial-3.html":
-      backButton.disabled = false;
-      nextButton.disabled = false;
+      backButton.hidden = false;
+      nextButton.hidden = false;
       break;
     case "tutorial-4.html":
-      backButton.disabled = false;
-      nextButton.disabled = true;
+      backButton.hidden = false;
+      nextButton.hidden = true;
       break;
     default:
   }
@@ -38,7 +38,7 @@ function next() {
 window.addEventListener("load", () => {
   let backButton = document.getElementById("backButton");
   let nextButton = document.getElementById("nextButton");
-  backButton.disabled = true;
+  backButton.hidden = true;
   // The following can't be done in the HTML file, see last section of:
   // eslint-disable-next-line max-len
   // https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Content_Security_Policy#Location_of_script_and_object_resources