Relanding without the test just to make sure this is in fact what's breaking stuff

content/xul/document/src/nsXULContentSink.cpp

@@ -959,9 +959,16 @@ XULContentSinkImpl::OpenTag(const PRUnichar** aAttributes,
         // even though it is ignored (the nsPrototypeScriptElement
         // has its own script-type).
         element->mScriptTypeID = nsIProgrammingLanguage::JAVASCRIPT;
-        // OpenScript will push the nsPrototypeScriptElement onto the 
-        // stack, so we're done after this.
-        return OpenScript(aAttributes, aLineNumber);
+        rv = OpenScript(aAttributes, aLineNumber);
+        NS_ENSURE_SUCCESS(rv, rv);
+
+        NS_ASSERTION(mState == eInScript || mState == eInDocumentElement,
+                     "Unexpected state");
+        if (mState == eInScript) {
+            // OpenScript has pushed the nsPrototypeScriptElement onto the 
+            // stack, so we're done.
+            return NS_OK;
+        }
     }
 
     // Set the correct script-type for the element.
@@ -1004,7 +1011,15 @@ XULContentSinkImpl::OpenScript(const PRUnichar** aAttributes,
           rv = mimeHdrParser->GetParameter(typeAndParams, nsnull,
                                            EmptyCString(), PR_FALSE, nsnull,
                                            mimeType);
-          NS_ENSURE_SUCCESS(rv, rv);
+          if (NS_FAILED(rv)) {
+              if (rv == NS_ERROR_INVALID_ARG) {
+                  // Might as well bail out now instead of setting langID to
+                  // nsIProgrammingLanguage::UNKNOWN and bailing out later.
+                  return NS_OK;
+              }
+              // We do want the warning here
+              NS_ENSURE_SUCCESS(rv, rv);
+          }
 
           // Javascript keeps the fast path, optimized for most-likely type
           // Table ordered from most to least likely JS MIME types. For .xul

content/xul/document/src/nsXULContentSink.h

@@ -105,6 +105,12 @@ protected:
                      const PRUint32 aLineNumber,
                      nsINodeInfo *aNodeInfo);
 
+    // If OpenScript returns NS_OK and after it returns our state is eInScript,
+    // that means that we created a prototype script and stuck it on
+    // mContextStack.  If NS_OK is returned but the state is still
+    // eInDocumentElement then we didn't create a prototype script (e.g. the
+    // script had an unknown type), and the caller should create a prototype
+    // element.
     nsresult OpenScript(const PRUnichar** aAttributes,
                         const PRUint32 aLineNumber);