--- a/browser/base/content/browser.css
+++ b/browser/base/content/browser.css
@@ -357,18 +357,18 @@ window[chromehidden~="toolbar"] toolbar:
   background: black;
 }
 
 #full-screen-warning-container {
   pointer-events: none;
   position: fixed;
   top: 0;
   left: 0;
-  min-width: 100%;
-  min-height: 100%;
+  width: 100%;
+  height: 100%;
 }
 
 #full-screen-warning-container[fade-warning-out] {
   -moz-transition-property: opacity !important;
   -moz-transition-duration: 500ms !important;
   opacity: 0.0;
 }
 

--- a/browser/base/content/browser.xul
+++ b/browser/base/content/browser.xul
@@ -971,17 +971,17 @@
           style="min-width: 18em; width: 22em; max-width: 42em;" persist="width">
       <toolbar id="devtools-sidebar-toolbar" nowindowdrag="true"/>
       <deck id="devtools-sidebar-deck" flex="1"/>
     </vbox>
     <vbox id="browser-border-end" hidden="true" layer="true"/>
   </hbox>
 
   <hbox id="full-screen-warning-container" hidden="true" fadeout="true">
-    <hbox style="min-width: 100%;" pack="center"> <!-- Inner hbox needed due to bug 579776. -->
+    <hbox style="width: 100%;" pack="center"> <!-- Inner hbox needed due to bug 579776. -->
       <hbox id="full-screen-warning-message">
         <description id="full-screen-warning-text" value="&domFullScreenWarning.label;"></description>
       </hbox>
     </hbox>
   </hbox>
 
   <vbox id="browser-bottombox" layer="true">
     <toolbar id="inspector-toolbar"