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

Table Of Contents

• Firefox can't access secure sites
• Incorrect update URL setting
• Upgrading manually

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. From the menu at the top of the Firefox windowbar, select FileFirefox and then select the ExitQuit FirefoxQuit menu item.
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 Error loading web sites.
• 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. In the Location bar, type about:config, and press EnterReturn.
   • The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise!, to continue 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.

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.