I don't understand what BES has to do with not running IE... Once you run enterprise activation, your desktop computer plays no role...
If you want better firefox support, find a provider who uses Lotus Domino on the backend for mail and go with them. Microsoft isn't going to go out of their way to make OWA work in other browsers. As far as I knew, application developers were responsible for making their apps work with various services, not the other way around.
AFAIK neither Notes nor Outlook let you have email reminders for calendar events, just the standard reminders.