If you mean the AJAX client, there is no such "page". The entire AJAX app is one huge amalgamation of JavaScript code. It's written to the MVC paradigm, which means that there are .
js files corresponding to the model, view and controller for compose:
ZmComposeController.js,
ZmComposeView.js, and
ZmMailMsg.js. These .
js files aren't standalone, but they're probably the best place to start looking as long as you have a very good grasp of JavaScript.
If you mean the HTML "lite" client, just use the path in the URL ("
h/compose") to find the relevant JSP page that generates it. In the source tree, it's
ZimbraWebClient/WebRoot/h/compose. Note again that this is a JSP page and not a straight HTML file, so you're going to need to understand JSP and the Zimbra taglib before trying to modify it.