There is a preference in Azureus Vuze to "Force Vuze to use Mozilla for Browser widgets [xulrunner or Firefox 3 required; restart required" When Vuze is running and this is set, assuming the path specified to Firefox.exe is filled in, trying to update Firefox will keep failing saying that Firefox is already running. Quitting Vuze allows the process to complete. Even the full-installer will fail. I have updated the Support wiki. I think that the installer should figure out what is preventing the update and notify the user.