--- a/browser/base/content/browser.xul
+++ b/browser/base/content/browser.xul
@@ -1046,17 +1046,17 @@
     <hbox flex="1" id="browser">
       <vbox id="browser-border-start" hidden="true" layer="true"/>
       <vbox id="sidebar-box" hidden="true" class="chromeclass-extrachrome">
         <sidebarheader id="sidebar-header" align="center">
           <label id="sidebar-title" persist="value" flex="1" crop="end" control="sidebar"/>
           <image id="sidebar-throbber"/>
           <toolbarbutton class="close-icon tabbable" tooltiptext="&sidebarCloseButton.tooltip;" oncommand="SidebarUI.hide();"/>
         </sidebarheader>
-        <browser id="sidebar" flex="1" autoscroll="false" disablehistory="true"
+        <browser id="sidebar" flex="1" autoscroll="false" disablehistory="true" disablefullscreen="true"
                   style="min-width: 14em; width: 18em; max-width: 36em;" tooltip="aHTMLTooltip"/>
       </vbox>
 
       <splitter id="sidebar-splitter" class="chromeclass-extrachrome sidebar-splitter" hidden="true"/>
       <vbox id="appcontent" flex="1">
         <notificationbox id="high-priority-global-notificationbox" notificationside="top"/>
         <tabbrowser id="content"
                     flex="1" contenttooltip="aHTMLTooltip"

--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -2501,50 +2501,57 @@ nsDocShell::GetFullscreenAllowed(bool* a
 
   // Assume false until we determine otherwise...
   *aFullscreenAllowed = false;
 
   nsCOMPtr<nsPIDOMWindowOuter> win = GetWindow();
   if (!win) {
     return NS_OK;
   }
-  nsCOMPtr<Element> frameElement = win->GetFrameElementInternal();
-  if (frameElement && !frameElement->IsXULElement()) {
-    // We do not allow document inside any containing element other
-    // than iframe to enter fullscreen.
-    if (frameElement->IsHTMLElement(nsGkAtoms::iframe)) {
-      // If any ancestor iframe does not have allowfullscreen attribute
-      // set, then fullscreen is not allowed.
-      if (!frameElement->HasAttr(kNameSpaceID_None,
-                                 nsGkAtoms::allowfullscreen) &&
-          !frameElement->HasAttr(kNameSpaceID_None,
-                                 nsGkAtoms::mozallowfullscreen)) {
-        return NS_OK;
-      }
-    } else if (frameElement->IsHTMLElement(nsGkAtoms::embed)) {
-      // Respect allowfullscreen only if this is a rewritten YouTube embed.
-      nsCOMPtr<nsIObjectLoadingContent> objectLoadingContent =
-        do_QueryInterface(frameElement);
-      if (!objectLoadingContent) {
-        return NS_OK;
-      }
-      nsObjectLoadingContent* olc =
-        static_cast<nsObjectLoadingContent*>(objectLoadingContent.get());
-      if (!olc->IsRewrittenYoutubeEmbed()) {
-        return NS_OK;
-      }
-      // We don't have to check prefixed attributes because Flash does not
-      // support them.
-      if (!frameElement->HasAttr(kNameSpaceID_None,
-                                 nsGkAtoms::allowfullscreen)) {
+  if (nsCOMPtr<Element> frameElement = win->GetFrameElementInternal()) {
+    if (frameElement->IsXULElement()) {
+      if (frameElement->HasAttr(kNameSpaceID_None,
+                                nsGkAtoms::disablefullscreen)) {
+        // Document inside this frame is explicitly disabled.
         return NS_OK;
       }
     } else {
-      // neither iframe nor embed
-      return NS_OK;
+      // We do not allow document inside any containing element other
+      // than iframe to enter fullscreen.
+      if (frameElement->IsHTMLElement(nsGkAtoms::iframe)) {
+        // If any ancestor iframe does not have allowfullscreen attribute
+        // set, then fullscreen is not allowed.
+        if (!frameElement->HasAttr(kNameSpaceID_None,
+                                  nsGkAtoms::allowfullscreen) &&
+            !frameElement->HasAttr(kNameSpaceID_None,
+                                  nsGkAtoms::mozallowfullscreen)) {
+          return NS_OK;
+        }
+      } else if (frameElement->IsHTMLElement(nsGkAtoms::embed)) {
+        // Respect allowfullscreen only if this is a rewritten YouTube embed.
+        nsCOMPtr<nsIObjectLoadingContent> objectLoadingContent =
+          do_QueryInterface(frameElement);
+        if (!objectLoadingContent) {
+          return NS_OK;
+        }
+        nsObjectLoadingContent* olc =
+          static_cast<nsObjectLoadingContent*>(objectLoadingContent.get());
+        if (!olc->IsRewrittenYoutubeEmbed()) {
+          return NS_OK;
+        }
+        // We don't have to check prefixed attributes because Flash does not
+        // support them.
+        if (!frameElement->HasAttr(kNameSpaceID_None,
+                                  nsGkAtoms::allowfullscreen)) {
+          return NS_OK;
+        }
+      } else {
+        // neither iframe nor embed
+        return NS_OK;
+      }
     }
   }
 
   // If we have no parent then we're the root docshell; no ancestor of the
   // original docshell doesn't have a allowfullscreen attribute, so
   // report fullscreen as allowed.
   RefPtr<nsDocShell> parent = GetParentDocshell();
   if (!parent) {

--- a/dom/base/nsGkAtomList.h
+++ b/dom/base/nsGkAtomList.h
@@ -307,16 +307,17 @@ GK_ATOM(digit, "digit")
 GK_ATOM(dir, "dir")
 GK_ATOM(dirAutoSetBy, "dirAutoSetBy")
 GK_ATOM(directionality, "directionality")
 GK_ATOM(directory, "directory")
 GK_ATOM(disableOutputEscaping, "disable-output-escaping")
 GK_ATOM(disabled, "disabled")
 GK_ATOM(disableglobalhistory, "disableglobalhistory")
 GK_ATOM(disablehistory, "disablehistory")
+GK_ATOM(disablefullscreen, "disablefullscreen")
 GK_ATOM(display, "display")
 GK_ATOM(displayMode, "display-mode")
 GK_ATOM(distinct, "distinct")
 GK_ATOM(div, "div")
 GK_ATOM(dl, "dl")
 GK_ATOM(doctypePublic, "doctype-public")
 GK_ATOM(doctypeSystem, "doctype-system")
 GK_ATOM(document, "document")