If you experience some of the following problems:

• Firefox never updates automatically
• When you click Check for updates from the Help menu, you get the error message "AUS: Update XML File Not Found (404)"
• Add-ons never update automatically
• When you click Find Updates from the Add-ons window, you get the error message "Some problems were encountered when trying to find updates."
• You cannot install some add-ons (not all), and you get the error message "The add-on could not be installed because it is not compatible with Firefox.", but you know they are compatible.

These problems might be caused by a program called "TDC Digital Signatur" installed on your computer.

Removing this program will most likely help.

1. In Windows, click on the Start button and select Control Panel to open the Windows Control Panel.
2. In the Windows Control Panel, double-click on Add or remove programs.
3. In the list of programs, select TDC Digital Signatur, and click Remove.

For more information and options on how to fix this problem while still keeping your digital signatures, please see this Firefox Support page (written in Danish).