This has been implemented for IMP 4.2.1.

attached patch touches two files:

1)  IMP.php  function: getComposeArgs
      mailto URI refers to the message body as "body", not as "message".

2)  The added HTML file registers the mailto handler for FF3
      (has to be called from same domain as IMP, see mentioned FF3 docs)
      Keep the %26 for & in the handler URL!


A security thought: I propose to strip CC and BCC from the mailto: URL 
(i.e. don't copy them in getComposeArgs, if this function is not used 
otherwise too), as these fields are not always shown in the compose 
popup and the (normal) user would not expect these copies. From RFC 
2368 (The mailto URL scheme):

4. Unsafe headers
    The user agent interpreting a mailto URL SHOULD choose not to create
    a message if any of the headers are considered dangerous; it may also
    choose to create a message with only a subset of the headers given in
    the URL.  Only the Subject, Keywords, and Body headers are believed
    to be both safe and useful.

    The creator of a mailto URL cannot expect the resolver of a URL to
    understand more than the "subject" and "body" headers.


Martin

Add support for Firefox 3 web based protocol handlers:
http://developer.mozilla.org/en/docs/Web-based_protocol_handlers

Probably also useful for other apps as well.