Sometimes Windows security doesn't allow UA server to update the registry. 

If this is your case, please follow the below procedure to update it manually:

UA Server writes to the registry on your machine. It updates HKLM\Mozilla\NativeMessagingHosts\ua_firefoxhost:

Please make sure that the value is set to the user folder of the user that you are logged into.

UA Server also copies the file to that location:

Please confirm that the file has been written to the location of the user that is currently logged in. 

Our expectation is that the file is in the right place but we may be prevented from updating the registry on your machine. In that case you can make the registry setting manually. Then restart everything. Sometimes you have to reboot the machine after updating the registry.

In addition please access 'User Account Control Settings' and pull the bar down to the bottom.