Windows vista error while updating firefox
For example, on Windows 7, the files "active-update.xml", "updates.xml", and the "updates" folder can be found in one of these locations  (using Firefox in the first example).
Note: You'll need to show hidden files and folders to find these locations: A Software Update "Update Failed" error, Something is trying to trick Firefox into accepting an insecure update.
If the issue is still not resolved and you are using Windows Vista, try the following:  Another possible solution is to reset the Software Update feature by ending all Firefox processes and then removing "active-update.xml", "updates.xml" and the "updates" folder from the appropriate folder locations (see above).
As a last resort, download the latest version of Firefox from install it over your current version.
The message, AUS: Update XML File Malformed (200) can sometimes appear during a check for updates.
This is a "catch-all" update error which can occur for different reasons such as being in "offline" mode, network problems, or not being signed on with your Master password (if you've set one up) before the update check .
This article describes how to configure and run Software Update and discusses problems that can occur during or after an update.
When an update is found, follow the on-screen instructions to download and install the update.
https://addons.mozilla.org) see Error loading secure sites. If Software Update errors persist, you can download the latest version of Firefox or Thunderbird from install it "on top of" your current version, instead of using the Software Update feature.
Related bug reports include: Updates can be retrieved from a number of different update channels.
To check which channel you are on, look in about:config at app.update.channel.
Please make sure there are no other copies of Firefox running on your computer, and then restart Firefox to try again.
If you see this dialog, end any Firefox processes (or reboot the computer) and restart Firefox.