When I click Send, it appears that GMail is sending the email (the "Sending... Cancel" message appears on top of the screen) and it stays this way forever. The screen never refreshes and GMail appears to be stuck. This seems to be a UI problem because I've confirmed the email is indeed sent.
This has been happening to me ever since I upgraded to Beta 4. It happens every time on two different Windows 7 PCs.
Sending GMail in IE7 works as expected but seems broken in Firefox 4.0b4.