I just started using the Zimbra Toaster app that is packaged with Zimbra Desktop. While it's nice in it's concept, it isn't consistent and has a shortcoming or two. Perhaps your Firefox extension could fill in the blanks.
First, the mail check that is does continues to show messages as being new, even though I've deleted the message. This pretty much defeats the purpose of new mail notification.
Second, and perhaps more on target with your extension capabilities, is the mailto registration. I absolutely LOVE the fact that it opens up a Zimbra session and starts to compose a message. That integration is exactly what I was hoping to have at some point. My disappointment lies within the fact that if I already have a Zimbra tab/session open it doesn't re-use the existing tab. It starts an entirely new one.
I use two Firefox extensions for Zimbra, as well as a few other "websites"; FaviconizeTab & Permanent Tab. Faviconize reduces a tab heading to just the icon, eliminating any text description. Permanent Tab prevents the tab from ever closing or any links from redirecting away from the associated site. Anything external will open in a new tab.
With that, I always have my Zimbra tab open and logged-in. If my computer is on, Firefox is open and Zimbra along with a few other sites are always open. I would be ecstatic if your plugin would make any mailto clicks use an existing Zimbra session/tab if one is already open.
BTW I am using the AJAX client
