You seem to be trying to install a Microsoft XAML/ClickOnce application - these require that you have the Microsoft .NET Framework 3.5 and/or Microsoft .NET Framework 4 installed. If you don't have any of these, go to Windows Update and install from there first, these are to the best of my knowledge absolutely free updates provided by Microsoft for Windows XP and newer versions. I am pretty sure that this page can help you get it working in Firefox too, if you already have a fully updated .NET Framework installed: [http://msdn.microsoft.com/en-us/library/cc716877.aspx Firefox Add-ons to Support .NET Application Deployment] If you still cannot get it working even thou you're sure you have the required .NET Framework (minimum 3.5 SP1) installed, you can manually enable the Microsoft plugin in Firefox: In Firefox, open menu Tools -> Add-ons... -> tab Plugins -> locate Windows Presentation Foundation (WPF) and click the Activate button. This is known to cause instability in Firefox, so you may want to Deactivate the plugin again after installing the application. Please let us know how above works out, and if we can be of further assistance. Detailed feedback is appreciated and helps us to help you better, plus enables us to better help other users with similar issues.