FindWindow 2 classes

Hi.....i want to do a check for FindWindow for two classes MozillaWindowClass and MozillaUIWindowClass at the same time.....or may be put an or condition .....but I'm not getting the correct syntax.......Plz help me out.......thnx in advance.....

FindWindow $0 MozillaWindowClass
IsWindow $0 +3
FindWindow $0 MozillaUIWindowClass
IsWindow $0 0 +3
MessageBox MB_OK "Please close firefox before installing."
Abort