--- a/dom/jsurl/nsJSProtocolHandler.cpp
+++ b/dom/jsurl/nsJSProtocolHandler.cpp
@@ -258,16 +258,21 @@ nsresult nsJSThunk::EvaluateScript(nsICh
     rv = principal->Subsumes(objectPrincipal, &subsumes);
     if (NS_FAILED(rv))
         return rv;
 
     if (!subsumes) {
         return NS_ERROR_DOM_RETVAL_UNDEFINED;
     }
 
+    // Fail if someone tries to execute in a global with system principal.
+    if (nsContentUtils::IsSystemPrincipal(objectPrincipal)) {
+        return NS_ERROR_DOM_SECURITY_ERR;
+    }
+
     JS::Rooted<JS::Value> v (cx, JS::UndefinedValue());
     // Finally, we have everything needed to evaluate the expression.
     JS::CompileOptions options(cx);
     options.setFileAndLine(mURL.get(), 1)
            .setVersion(JSVERSION_DEFAULT);
     nsJSUtils::EvaluateOptions evalOptions(cx);
     evalOptions.setCoerceToString(true);
     rv = nsJSUtils::EvaluateString(cx, NS_ConvertUTF8toUTF16(script),