Firefox on the Mac and with Windows tends to work best.
We have had good success with Camino on the Mac, too.
We have had mixed results with Safari & Chrome.
Due to general ongoing security and compatibility issues, we don’t recommend using Internet Explorer for anything unless you must.
As a side note, we have some customers who never use the web to access their mail accounts, instead always using a mail client, such as Outlook, Entourage, Thunderbird, Apple Mail, Eudora, and others, which are securely supported.