The Firefox error AUS: Update XML File Malformed (200) is a general update error message that can not be attributed to one specific issue.
Generally speaking, the error has to do with either the offline mode, proxy servers, network connections or https. Those options are but a few of the possibilities but common ones.
I'd like to provide you with information on how I fixed the issue on my version of Firefox which threw the error one day.
You can of course check all the settings mentioned above which will probably lead to the setting or element that causes the error AUS: Update XML File Malformed (200) to appear.
I would propose two options instead which update Firefox without having to uncover the cause of the error message at all.
The first that comes to my mind is simply download the newest version of Firefox from Mozilla and install it over the old version. This updates Firefox to the latest version.
Another possibility - if you want to use the Update Checker - is to create a new profile in Firefox and use that profile to update Firefox.
The second method won't work all the time depending on the cause of the error message but it can help if the error is caused by options, extensions or settings of the current Firefox profile.
To create a new profile under Windows for Firefox you need to close Firefox completely and start it with the parameter Firefox -p which will open the profile manager on start.
Once displayed, click on Create Profile and uncheck don't ask at startup. Start Firefox and select the new profile and use it to update the browser.
It is usually a good idea to have a second profile configured in the browser for situations where you need to troubleshoot issues that you are experiencing while using the main profile.
Anyway, the manual download and update update should work regardless of that.
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