How do you "run [your] PHP script in Firefox 4" ? Have you checked a phpinfo() readout to see what the PHP directives error_reporting and display_errors are set to? What are those values?
As for the MySQL issue, either look in the Windows Event Viewer after you receive the error message and try to find related error messages that give some useful debugging information, or try starting the MySQL daemon manually from the command line to see if MySQL offers any additional information.