This article describes why you can get AUS: Update XML File Malformed (200) when Firefox checks for updates.

Interfering extension

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 can't access secure sites

Firefox uses secure sites when it checks for updates. Make sure your Firefox is able to access secure sites without errors or prompts.

1. Click the Firefox menu and select Exit.Click the Firefox menu at the top of the screen and select Quit Firefox.Click the Firefox menu and select Quit.
2. Start Firefox.
3. In the Location Bar, type https://addons.mozilla.org and press EnterReturn.

• If you get an error loading the site, see Websites don't load - troubleshoot and fix error messages.
• If you get a prompt for your master password or for a proxy password, then Firefox is not able to check for updates until you receive that prompt and enter the correct password. As a workaround, you can periodically check for updates manually after you've entered your password.

Incorrect update URL setting

Firefox may be set to use an incorrect address when checking for updates.

1. Type about:config in the address bar and press EnterReturn.
   A warning page may appear. Click Accept the Risk and Continue to go to the about:config page.
2. Search for the preference app.update.url.
3. Right-clickHold down the Ctrl key while you click app.update.url and click Reset.

• • If Reset is disabled, Firefox is already set to use the correct update address, so this isn't your problem.

Now that Firefox is configured with the correct address, it may be able to update without error.

Cookies - Incorrectly set or Cannot be set

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.

1. Temporarily disable Stealther and/or other Privacy and Security add-ons.
2. In the Menu bar at the top of the screen, click Firefox and then select Preferences or Settings, depending on your macOS version.Click the menu button and select Settings.
3. Select the Privacy panel.
4. Click the Show Cookies... button to open the Cookies window.
5. Find a cookie beginning with the name aus2. If it exists, select the site and click Remove Cookie(s).
   
   
   

Eset security software

If you use Eset security software, it is known to cause this error because of the way it checks secure connections. For instructions on how to fix the problem, read this Eset article.

Upgrading manually

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.