The shell script is:
#!/bin/sh
exec /Applications/Mozilla.app/Contents/MacOS/mozilla-bin -mail
In fact, if you install this in your command path as mozmail, you can start Mozilla in email mode from the command line.
Platypus is an essential free application that takes a script and builds a Cocoa wrapper around it. I learned about it here on macosxhints. Anyway, it is trivial to create the clickable version of Mozmail.app with Platypus. You just fill in the name of the application you are creating and the path to the script, and click the "Create" button.
Finally, I added a nifty icon from Fablue to give it a distinctive character, and ran it through StuffIt to make it downloadable. Just download, unstuff, and move into your Applications directory or anywhere else. Or, you could get Platypus and do it yourself -- the whole process took literally less than one minute.
It assumes that Mozilla is installed in the location above; this can be changed by editing a file called script, which is within the application's folder.

