--- a/remote/domains/parent/Page.jsm
+++ b/remote/domains/parent/Page.jsm
@@ -115,17 +115,22 @@ class Page extends Domain {
     const ctx = canvas.getContext("2d");
     ctx.drawImage(snapshot, 0, 0);
 
     // Bug 1574935 - Huge dimensions can trigger an OOM because multiple copies
     // of the bitmap will exist in memory. Force the removal of the snapshot
     // because it is no longer needed.
     snapshot.close();
 
-    return canvas.toDataURL();
+    const url = canvas.toDataURL();
+
+    // only return the base64 encoded data without the data URL prefix
+    const data = url.substring(url.indexOf(",") + 1);
+
+    return { data };
   }
 
   async enable() {
     if (this.enabled) {
       return;
     }
 
     this.enabled = true;

--- a/remote/test/browser/page/browser_captureScreenshot.js
+++ b/remote/test/browser/page/browser_captureScreenshot.js
@@ -4,72 +4,71 @@
 "use strict";
 
 async function getDevicePixelRatio() {
   return ContentTask.spawn(gBrowser.selectedBrowser, null, function() {
     return content.devicePixelRatio;
   });
 }
 
-async function getImageDetails(client, image) {
-  return ContentTask.spawn(gBrowser.selectedBrowser, image, async function(
-    image
-  ) {
-    let infoPromise = new Promise(resolve => {
-      const img = new content.Image();
-      img.addEventListener(
-        "load",
-        () => {
-          resolve({
-            width: img.width,
-            height: img.height,
-          });
-        },
-        { once: true }
-      );
-      img.src = image;
-    });
-    return infoPromise;
-  });
+async function getImageDetails(format, data) {
+  return ContentTask.spawn(
+    gBrowser.selectedBrowser,
+    { format, data },
+    async function({ format, data }) {
+      return new Promise(resolve => {
+        const img = new content.Image();
+        img.addEventListener(
+          "load",
+          () => {
+            resolve({
+              width: img.width,
+              height: img.height,
+            });
+          },
+          { once: true }
+        );
+
+        img.src = `data:image/${format};base64,${data}`;
+      });
+    }
+  );
 }
 
 async function getViewportRect() {
   return ContentTask.spawn(gBrowser.selectedBrowser, null, function() {
     return {
       left: content.pageXOffset,
       top: content.pageYOffset,
       width: content.innerWidth,
       height: content.innerHeight,
     };
   });
 }
 
-add_task(async function documentSmallerThanViewport(client) {
+add_task(async function documentSmallerThanViewport({ Page }) {
   loadURL(toDataURL("<div>Hello world"));
 
-  const { Page } = client;
-
   info("Check that captureScreenshot() captures the viewport by default");
-  const screenshot = await Page.captureScreenshot();
+  const { data } = await Page.captureScreenshot();
+  ok(!!data, "Screenshot data is not empty");
 
   const scale = await getDevicePixelRatio();
   const viewportRect = await getViewportRect();
-  const { width, height } = await getImageDetails(client, screenshot);
-
+  const { width, height } = await getImageDetails("png", data);
   is(width, (viewportRect.width - viewportRect.left) * scale);
   is(height, (viewportRect.height - viewportRect.top) * scale);
 });
 
-add_task(async function documentLargerThanViewport(client) {
+add_task(async function documentLargerThanViewport({ Page }) {
   loadURL(toDataURL("<div style='margin: 100vh 100vw'>Hello world"));
 
-  const { Page } = client;
-
   info("Check that captureScreenshot() captures the viewport by default");
-  const screenshot = await Page.captureScreenshot();
+  const { data } = await Page.captureScreenshot();
+  ok(!!data, "Screenshot data is not empty");
 
   const scale = await getDevicePixelRatio();
   const viewportRect = await getViewportRect();
-  const { width, height } = await getImageDetails(client, screenshot);
+  const { width, height } = await getImageDetails("png", data);
 
   is(width, (viewportRect.width - viewportRect.left) * scale);
   is(height, (viewportRect.height - viewportRect.top) * scale);
 });