I started uploading builds of Composer here and I will do that as often as I can. The files there are xulrunner apps, so just use the last xulrunner release and type xulrunner --install-app filename.app to install it. Vendor name is "Disruptive Innovations SARL", product name is "Composer". Path is usually c:\Program Files\vendorname\productname\ on windows.

You can also unzip the *.app package and call xulrunner --app application.ini .

Warning, these are very very VERY early builds, there is almost nothing to see. The files do NOT reflect the state of cvs.mozilla.org, they reflect the state of my own work tree. Don't say I did not warn you...