Some years ago Julien Couvreur wrote an extension that I continue to use with minor changes. It solves the "mailto:" issue quite neatly.
I enclose the zip file. This is the procedure (on Linux) to adapt it to your FF2 or FF3. You can do the same thing on Windows of course. The only issue is to rename the final file from .zip to .xpi.
1) Create a work directory and unzip the file
mkdir mailto
unzip mailtotozimbra.zip
2) Edit the file "content/mailtotozimbra.
js" and change my web site by yours:
Zimbra Collaboration Suite Log In ->
http://whatever.../
3) Re-zip with:
zip ../mailtotozimbra.xpi .
4) Open Firefox, go to the menu "File->Open" and select the xpi file you have just built.
The extension will get installed.
With a little bit of work the extension can be generalized so that it asks for the URI upon installation. I didn't had time to do this, so I normally modify the script for each new customer ;-)
The advantage of this method is that you can handle the "mailto:" information in Javascript and extract To, Cc, Subject and Body. That's all what this extension does.