To offer new sidebar content to Composer users, you have to distribute your application as an installable extension to Composer, a *.xpi file.
The application id for Composer is
{8fa6f1b4-7ed3-4895-bac6-01f1cc206ab3}
.
Your extension should add elements to chrome://composer/content/composer.xul through a XUL overlay :
sidebaritem
element to
<sidebaritems id="sidebaritems">
name
attribute,
representing your sidebar content ; that sidebar should be a
NAME ; it is recommended that the ID and that attribute have
the same valuesrc
attribute,
containing the URL of the document to be shown in your
sidebar content; this is typically going to be a chrome URL
in your XPItitle
containing a (possibly localized) string representing the
sidebar item. That string will be used in the sidebar to list
the available sidebar items or title the visible sidebar
items. If absent, the value of the name
attribute is used.TBD