--- a/browser/tools/mozscreenshots/browser_boundingbox.js
+++ b/browser/tools/mozscreenshots/browser_boundingbox.js
@@ -1,27 +1,28 @@
 /* Any copyright is dedicated to the Public Domain.
  * http://creativecommons.org/publicdomain/zero/1.0/
  */
 
 "use strict";
 
+const scale = window.QueryInterface(Ci.nsIInterfaceRequestor)
+                    .getInterface(Ci.nsIDocShell).QueryInterface(Ci.nsIBaseWindow)
+                    .devicePixelsPerDesktopPixel;
+
 add_task(async function() {
-  const scale = window.QueryInterface(Ci.nsIInterfaceRequestor)
-                      .getInterface(Ci.nsIDocShell).QueryInterface(Ci.nsIBaseWindow)
-                      .devicePixelsPerDesktopPixel;
   let {bounds, rects} = TestRunner._findBoundingBox(["#tabbrowser-tabs"]);
-  let element = document.querySelector("#tabbrowser-tabs");
-  let tabBar = element.ownerDocument.getBoxObjectFor(element);
+  let tabbrowserTabs = document.querySelector("#tabbrowser-tabs");
+  let tabbrowserTabsBO = document.getBoxObjectFor(tabbrowserTabs);
 
   // Calculate expected values
-  let expectedLeft = scale * (tabBar.screenX - TestRunner.croppingPadding);
-  let expectedTop = scale * (tabBar.screenY - TestRunner.croppingPadding);
-  let expectedRight = scale * (tabBar.width + TestRunner.croppingPadding * 2) + expectedLeft;
-  let expectedBottom = scale * (tabBar.height + TestRunner.croppingPadding * 2) + expectedTop;
+  let expectedLeft = scale * (tabbrowserTabsBO.screenX - TestRunner.croppingPadding);
+  let expectedTop = scale * (tabbrowserTabsBO.screenY - TestRunner.croppingPadding);
+  let expectedRight = scale * (tabbrowserTabsBO.width + TestRunner.croppingPadding * 2) + expectedLeft;
+  let expectedBottom = scale * (tabbrowserTabsBO.height + TestRunner.croppingPadding * 2) + expectedTop;
 
   // Calculate browser region
   let windowLeft = window.screenX * scale;
   let windowTop = window.screenY * scale;
   let windowRight = window.outerWidth * scale + windowLeft;
   let windowBottom = window.outerHeight * scale + windowTop;
 
   // Adjust values based on browser window
@@ -43,60 +44,73 @@ add_task(async function() {
     "Checking _findBoundingBox union.top and rect.top is the same for a single selector");
   is(bounds.bottom, rects[0].bottom,
     "Checking _findBoundingBox union.bottom and rect.bottom is the same for a single selector");
 
   let result = TestRunner._findBoundingBox(["#forward-button", "#TabsToolbar"]);
   bounds = result.bounds;
   rects = result.rects;
 
-  element = document.querySelector("#TabsToolbar");
-  let tabToolbar = element.ownerDocument.getBoxObjectFor(element);
-  element = document.querySelector("#forward-button");
-  let fButton = element.ownerDocument.getBoxObjectFor(element);
+  let tabsToolbar = document.querySelector("#TabsToolbar");
+  let tabsToolbarBO = document.getBoxObjectFor(tabsToolbar);
+  let forwardButton = document.querySelector("#forward-button");
+  let fButtonBO = document.getBoxObjectFor(forwardButton);
 
   // Calculate expected values
-  expectedLeft = scale * (Math.min(tabToolbar.screenX, fButton.screenX)
-                              - TestRunner.croppingPadding);
-  expectedTop = scale * (Math.min(tabToolbar.screenY, fButton.screenY)
-                              - TestRunner.croppingPadding);
-  expectedRight = scale * (Math.max(tabToolbar.width + tabToolbar.screenX,
-                                    fButton.width + fButton.screenX)
-                              + TestRunner.croppingPadding);
-  expectedBottom = scale * (Math.max(tabToolbar.height + tabToolbar.screenY,
-                                     fButton.height + fButton.screenY)
-                              + TestRunner.croppingPadding );
+  expectedLeft = scale * (Math.min(tabsToolbarBO.screenX, fButtonBO.screenX)
+                   - TestRunner.croppingPadding);
+  expectedTop = scale * (Math.min(tabsToolbarBO.screenY, fButtonBO.screenY)
+                  - TestRunner.croppingPadding);
+  expectedRight = scale * (Math.max(tabsToolbarBO.width + tabsToolbarBO.screenX,
+                                    fButtonBO.width + fButtonBO.screenX)
+                    + TestRunner.croppingPadding);
+  expectedBottom = scale * (Math.max(tabsToolbarBO.height + tabsToolbarBO.screenY,
+                                     fButtonBO.height + fButtonBO.screenY)
+                     + TestRunner.croppingPadding);
 
   // Adjust values based on browser window
   expectedLeft = Math.max(expectedLeft, windowLeft);
   expectedTop = Math.max(expectedTop, windowTop);
   expectedRight = Math.min(expectedRight, windowRight);
   expectedBottom = Math.min(expectedBottom, windowBottom);
 
   // Check width calculation on union
   is(bounds.width, expectedRight - expectedLeft,
      "Checking _findBoundingBox union width calculation");
   // Check height calculation on union
   is(bounds.height, expectedBottom - expectedTop,
      "Checking _findBoundingBox union height calculation");
   // Check single selector's left position
-  is(rects[0].left, Math.max(scale * (fButton.screenX - TestRunner.croppingPadding), windowLeft),
+  is(rects[0].left, Math.max(scale * (fButtonBO.screenX - TestRunner.croppingPadding), windowLeft),
     "Checking single selector's left position when _findBoundingBox has multiple selectors");
   // Check single selector's right position
-  is(rects[0].right, Math.min(scale * (fButton.width + fButton.screenX + TestRunner.croppingPadding), windowRight),
+  is(rects[0].right, Math.min(scale * (fButtonBO.width + fButtonBO.screenX + TestRunner.croppingPadding), windowRight),
     "Checking single selector's right position when _findBoundingBox has multiple selectors");
   // Check single selector's top position
-  is(rects[0].top, Math.max(scale * (fButton.screenY - TestRunner.croppingPadding), windowTop),
+  is(rects[0].top, Math.max(scale * (fButtonBO.screenY - TestRunner.croppingPadding), windowTop),
     "Checking single selector's top position when _findBoundingBox has multiple selectors");
   // Check single selector's bottom position
-  is(rects[0].bottom, Math.min(scale * (fButton.height + fButton.screenY + TestRunner.croppingPadding), windowBottom),
+  is(rects[0].bottom, Math.min(scale * (fButtonBO.height + fButtonBO.screenY + TestRunner.croppingPadding), windowBottom),
     "Checking single selector's bottom position when _findBoundingBox has multiple selectors");
 
     // Check that nonexistent selectors throws an exception
   Assert.throws(() => {
     TestRunner._findBoundingBox(["#does_not_exist"]);
   }, "No element for '#does_not_exist' found.", "Checking that nonexistent selectors throws an exception");
 
   // Check that no selectors throws an exception
   Assert.throws(() => {
     TestRunner._findBoundingBox([]);
   }, "No selectors specified.", "Checking that no selectors throws an exception");
 });
+
+add_task(function* testMaximizedWindow() {
+  window.maximize();
+
+  let {bounds, rects} = TestRunner._findBoundingBox([":root"]);
+
+  is(bounds.top, screen.availTop, "Checking _findBoundingBox top (" + bounds.top + ") calculation (maximized window)");
+  is(bounds.left, screen.availLeft, "Checking _findBoundingBox left (" + bounds.left + ") calculation (maximized window)");
+  is(bounds.width, screen.availWidth, "Checking _findBoundingBox width (" + bounds.width + ") calculation (maximized window)");
+  is(bounds.height, screen.availHeight, "Checking _findBoundingBox height (" + bounds.height + ") calculation (maximized window)");
+
+  window.restore();
+});

--- a/browser/tools/mozscreenshots/mozscreenshots/extension/TestRunner.jsm
+++ b/browser/tools/mozscreenshots/mozscreenshots/extension/TestRunner.jsm
@@ -228,22 +228,29 @@ this.TestRunner = {
 
       if (!element) {
         throw `No element for '${selector}' found.`;
       }
 
       // Calculate box region, convert to Rect
       let box = element.ownerDocument.getBoxObjectFor(element);
       let rect = new Rect(box.screenX * scale, box.screenY * scale,
-                             box.width * scale, box.height * scale);
+                          box.width * scale, box.height * scale);
       rect.inflateFixed(this.croppingPadding * scale);
       rect.left = Math.max(rect.left, windowLeft);
       rect.top = Math.max(rect.top, windowTop);
       rect.right = Math.min(rect.right, windowLeft + windowWidth);
       rect.bottom = Math.min(rect.bottom, windowTop + windowHeight);
+
+      // Clamp to available screen width/height
+      rect.left = Math.max(rect.left, browserWindow.screen.availLeft);
+      rect.top = Math.max(rect.top, browserWindow.screen.availTop);
+      rect.right = Math.min(rect.right, browserWindow.screen.availLeft + browserWindow.screen.availWidth);
+      rect.bottom = Math.min(rect.bottom, browserWindow.screen.availTop + browserWindow.screen.availHeight);
+
       rects.push(rect);
 
       if (!bounds) {
         bounds = rect;
       } else {
         bounds = bounds.union(rect);
       }
     }