Zimbra 8 fresh install, upgraded to 8.0.1 on ubuntu 12.04 LTS x64
Nothing happens trying to compose a new mail or replaying to a mail, it only works if I enable the option to compose new e-mails in a new windows.
Tested on Firefox 16 and IE 9 on windows 7, Firefox 16 on ubuntu 12.04 and Safari 5.1.7 on Mac Os X.
Reading some other posts here I've tried to disable all zimlets and flush zimlet cache, tried to launch upgrade again, tried to fix permission... but the problem remains.
It seems to be an AJAX problem, when I go to html mode all works correctly.