diff --git a/mobile/chrome/content/browser.js b/mobile/chrome/content/browser.js
index 6aa924ff4333..e5755fbd224d 100644
--- a/mobile/chrome/content/browser.js
+++ b/mobile/chrome/content/browser.js
@@ -2487,6 +2487,10 @@ Tab.prototype = {
       } else if (!validW && !validH) {
         viewportW = this.useFallbackWidth ? kFallbackBrowserWidth : kDefaultBrowserWidth;
         viewportH = kDefaultBrowserWidth * (screenH / screenW);
+
+        // Make sure the viewport height is not shorter than the window when
+        // the page is zoomed out to show its full width.
+        viewportH = Math.max(viewportH, screenH * (browser.contentDocumentWidth / screenW));
       }
     }