The built-in protocol handler broke for me and others with Firefox 3.6. If it works reliably for you with 3.6.x and 4.x, I'll take another look.
On Macs I suggest
Webmailer and on PCs put explorer "https://mail.carleton.edu/zimbra/?view=compose&to=%1" in HKEY_CLASSES_ROOT\mailto\shell\open\command (unfortunately requires admin rights).
The advantage of handling mailto: at the OS level is that it will work across browsers, and often (but not alway) in email programs.
To deal with the Windows "Send file to..." shell extension and a few other things, you'll need a purpose-build application.