This article describes why you can get AUS: Update XML File Malformed (200) when Firefox checks for updates.
Disable any extensions that may be blocking access to the update server, such as Noscript. For more information, see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.
Firefox uses secure sites when it checks for updates. Make sure your Firefox is able to access secure sites without errors or prompts.
Firefox may be set to use an incorrect address when checking for updates.
In the address bar, type about:config and press EnterReturn.
Now that Firefox is configured with the correct address, it may be able to update without error.
Firefox may not be able to set or read cookies when it checks for updates. Make sure your Firefox is able set or access cookies, especially when using Stealther and/or other Privacy and Security add-ons.
If nothing else works, you can always download the latest version of Firefox from http://www.getfirefox.com and install it over top of your existing version. You won't lose your bookmarks or other settings. To prevent the error dialog from coming up, you can turn off automatic updates.