diff --git a/testing/marionette/driver.js b/testing/marionette/driver.js
index 81b119c8777d..ee702a0d478a 100644
--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -3014,8 +3014,8 @@ GeckoDriver.prototype.takeScreenshot = async function(cmd) {
         let el = this.curBrowser.seenEls.get(webEl, win);
         rect = el.getBoundingClientRect();
       } else if (full) {
-        let clientRect = win.document.documentElement.getBoundingClientRect();
-        rect = new win.DOMRect(0, 0, clientRect.width, clientRect.height);
+        const docEl = win.document.documentElement;
+        rect = new DOMRect(0, 0, docEl.scrollWidth, docEl.scrollHeight);
       } else {
         // viewport
         rect = new win.DOMRect(
diff --git a/testing/marionette/harness/marionette_harness/tests/unit/test_screenshot.py b/testing/marionette/harness/marionette_harness/tests/unit/test_screenshot.py
index 9e9f4699ff0b..4956ed4883b6 100644
--- a/testing/marionette/harness/marionette_harness/tests/unit/test_screenshot.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_screenshot.py
@@ -218,7 +218,10 @@ class TestScreenCaptureContent(WindowManagerMixin, ScreenCaptureTestCase):
     @property
     def scroll_dimensions(self):
         return tuple(self.marionette.execute_script("""
-            return [document.body.scrollWidth, document.body.scrollHeight]
+            return [
+              document.documentElement.scrollWidth,
+              document.documentElement.scrollHeight
+            ];
             """))
 
     def test_capture_tab_already_closed(self):
@@ -266,18 +269,18 @@ class TestScreenCaptureContent(WindowManagerMixin, ScreenCaptureTestCase):
                          self.get_image_dimensions(screenshot))
         self.assertGreater(self.page_y_offset, 0)
 
-    def test_capture_html_document_element(self):
+    def test_capture_full_html_document_element(self):
         self.marionette.navigate(long)
         screenshot = self.marionette.screenshot()
         self.assert_png(screenshot)
         self.assertEqual(self.scale(self.scroll_dimensions),
                          self.get_image_dimensions(screenshot))
 
-    def test_capture_svg_document_element(self):
+    def test_capture_full_svg_document_element(self):
         self.marionette.navigate(svg)
         screenshot = self.marionette.screenshot()
         self.assert_png(screenshot)
-        self.assertEqual(self.scale(self.get_element_dimensions(self.document_element)),
+        self.assertEqual(self.scale(self.scroll_dimensions),
                          self.get_image_dimensions(screenshot))
 
     def test_capture_viewport(self):
diff --git a/testing/marionette/listener.js b/testing/marionette/listener.js
index 8d307803fb92..c6f621f0d1f3 100644
--- a/testing/marionette/listener.js
+++ b/testing/marionette/listener.js
@@ -1663,8 +1663,8 @@ function getScreenshotRect({ el, full = true, scroll = true } = {}) {
     }
     rect = getElementRect(el);
   } else if (full) {
-    let clientRect = win.document.documentElement.getBoundingClientRect();
-    rect = new DOMRect(0, 0, clientRect.width, clientRect.height);
+    const docEl = win.document.documentElement;
+    rect = new DOMRect(0, 0, docEl.scrollWidth, docEl.scrollHeight);
   } else {
     // viewport
     rect = new DOMRect(
diff --git a/testing/web-platform/mozilla/tests/webdriver/take_full_screenshot/__init__.py b/testing/web-platform/mozilla/tests/webdriver/take_full_screenshot/__init__.py
index fa86372a78e8..d5de0d731c20 100644
--- a/testing/web-platform/mozilla/tests/webdriver/take_full_screenshot/__init__.py
+++ b/testing/web-platform/mozilla/tests/webdriver/take_full_screenshot/__init__.py
@@ -1,7 +1,8 @@
 def document_dimensions(session):
     return tuple(session.execute_script("""
-        let {devicePixelRatio} = window;
-        let {width, height} = document.documentElement.getBoundingClientRect();
+        const {devicePixelRatio} = window;
+        const width = document.documentElement.scrollWidth;
+        const height = document.documentElement.scrollHeight;
 
         return [Math.floor(width * devicePixelRatio), Math.floor(height * devicePixelRatio)];
         """))