--- a/dom/base/nsObjectLoadingContent.cpp
+++ b/dom/base/nsObjectLoadingContent.cpp
@@ -1572,16 +1572,30 @@ nsObjectLoadingContent::UpdateObjectPara
   // already opened a channel or tried to instantiate content, whereas channel
   // parameter changes require re-opening the channel even if we haven't gotten
   // that far.
   nsObjectLoadingContent::ParameterUpdateFlags retval = eParamNoChange;
 
   ///
   /// Initial MIME Type
   ///
+
+
+  if (caps & eSupportClassID) {
+    nsAutoString classIDAttr;
+    thisElement->GetAttr(kNameSpaceID_None, nsGkAtoms::classid, classIDAttr);
+    if (!classIDAttr.IsEmpty()) {
+      // XXX(johns): Our de-facto behavior since forever was to refuse to load
+      // Objects who don't have a classid we support, regardless of other type
+      // or uri info leads to a valid plugin.
+      newMime.Truncate();
+      stateInvalid = true;
+    }
+  }
+
   ///
   /// Codebase
   ///
 
   nsAutoString codebaseStr;
   nsCOMPtr<nsIURI> docBaseURI = thisElement->GetBaseURI();
   thisElement->GetAttr(kNameSpaceID_None, nsGkAtoms::codebase, codebaseStr);
 

--- a/dom/base/nsObjectLoadingContent.h
+++ b/dom/base/nsObjectLoadingContent.h
@@ -298,18 +298,18 @@ class nsObjectLoadingContent : public ns
                         bool aForceLoad = false);
 
     enum Capabilities {
       eSupportImages       = 1u << 0, // Images are supported (imgILoader)
       eSupportPlugins      = 1u << 1, // Plugins are supported (nsIPluginHost)
       eSupportDocuments    = 1u << 2, // Documents are supported
                                         // (nsIDocumentLoaderFactory)
                                         // This flag always includes SVG
-      eSupportClassID      = 1u << 3, // The classid attribute is supported. No
-                                      // longer used.
+      eSupportClassID      = 1u << 3, // The classid attribute is supported. Used with
+                                      // flash.
 
       // If possible to get a *plugin* type from the type attribute *or* file
       // extension, we can use that type and begin loading the plugin before
       // opening a channel.
       // A side effect of this is if the channel fails, the plugin is still
       // running.
       eAllowPluginSkipChannel  = 1u << 4
     };

--- a/dom/html/HTMLObjectElement.cpp
+++ b/dom/html/HTMLObjectElement.cpp
@@ -514,17 +514,17 @@ EventStates
 HTMLObjectElement::IntrinsicState() const
 {
   return nsGenericHTMLFormElement::IntrinsicState() | ObjectState();
 }
 
 uint32_t
 HTMLObjectElement::GetCapabilities() const
 {
-  return nsObjectLoadingContent::GetCapabilities();
+  return nsObjectLoadingContent::GetCapabilities() | eSupportClassID;
 }
 
 void
 HTMLObjectElement::DestroyContent()
 {
   nsObjectLoadingContent::DestroyContent();
   nsGenericHTMLFormElement::DestroyContent();
 }