Hmm that is a good question! Here at Feng Office we run Linux, Windows and Macs we have never experienced this issue.
Any chance it is a server side problem?
Have you tried using your Mac with our Demo? Please do so: demo.fengoffice.com
If it works, it is something with your server.
If it does not, it could be something with your computer or may be with Feng (pretty odd as many people here use Macs too)
Best regards,
Francisco