So people, nice that this is working for Linux too now, but is there a possibility to turn it of? Sometimes, one wants to work without being disturbed... Switching mail.server.default.notify.on does not seem to do the job, and MozillaZine is outdated (http://kb.mozillazine.org/index.php?title=Disable_the_new_mail_system_tray_icon&printable=yes) See also bug 312940.
Yes. Preferences > General | When a new message arrives.... Feel free to update the kb article;)
Magnus showed you were in the UI you can turn this off..