Several people have reported that the Send button is not visible when writing an email message in the new version of Office 365 Outlook Web App (OWA). This behavior occurs with Firefox on both Windows and Mac computers, and has proven to be very inconsistent. If you are experiencing this problem you can try the follow workaround:
Now when writing a new message or replying to a message, the Send button will always be visible.
If this is not a suitable workaround, the only other reliable option is to use a different Web browser to access OWA.
Microsoft is aware of the problem.