Bug 1705799 - Switch multistage about:welcome background image to use avif r=pdahiya

Differential Revision: https://phabricator.services.mozilla.com/D133760

browser/components/newtab/aboutwelcome/lib/AboutWelcomeDefaults.jsm

@@ -21,7 +21,7 @@ const DEFAULT_WELCOME_CONTENT = {
   template: "multistage",
   transitions: true,
   background_url:
-    "chrome://activity-stream/content/data/content/assets/proton-bkg.jpg",
+    "chrome://activity-stream/content/data/content/assets/proton-bkg.avif",
   screens: [
     {
       id: "AW_PIN_FIREFOX",

browser/components/newtab/test/browser/browser_aboutwelcome_multistage_default.js

@@ -138,7 +138,7 @@ add_task(async function test_multistage_aboutwelcome_proton() {
     [
       "main.AW_STEP1",
       "div.onboardingContainer",
-      "div.proton[style*='.jpg']",
+      "div.proton[style*='.avif']",
       "div.section-left",
       "span.attrib-text",
       "div.secondary-cta.top",
@@ -179,7 +179,7 @@ add_task(async function test_multistage_aboutwelcome_proton() {
     [
       "main.AW_STEP2.dialog-initial",
       "div.onboardingContainer",
-      "div.proton[style*='.jpg']",
+      "div.proton[style*='.avif']",
       "div.section-main",
       "nav.steps",
       "div.indicator.current",
@@ -200,7 +200,7 @@ add_task(async function test_multistage_aboutwelcome_proton() {
     [
       "main.AW_STEP3",
       "div.onboardingContainer",
-      "div.proton[style*='.jpg']",
+      "div.proton[style*='.avif']",
       "div.section-main",
       "div.tiles-theme-container",
       "nav.steps",

browser/components/newtab/test/unit/aboutwelcome/MultiStageAWProton.test.jsx

@@ -150,7 +150,7 @@ describe("MultiStageAboutWelcomeProton module", () => {
         id: "DEFAULT_ABOUTWELCOME_PROTON",
         template: "multistage",
         transitions: true,
-        background_url: `chrome://activity-stream/content/data/content/assets/proton-bkg.jpg`,
+        background_url: `chrome://activity-stream/content/data/content/assets/proton-bkg.avif`,
         screens: [
           {
             id: "AW_PIN_FIREFOX",