Jason,

 Thanks for the suggestion - I don't know whether it would have any effect, but loading the compose page in Firefox Portable as above creates a log in the Error Console:

 document.write is not a function
http://server.com/plugins/html_mail/tiny_mce/tiny_mce_gzip.js
Line 128

I think I remember something about document.write not being supported in XHTML Strict - can't remember exactly though since I'm not good at Javascript.

 The second problem of the drop-down boxes not functioning doesn't generate any error messages in the console or any where else that I can think of. To be more specific: on a mailbox page, selecting "Show Unread Messages" causes "Filtering mailbox..." to be displayed at the top.  Then nothing happens. The page isn't loading, the filtering message doesn't go away, nothing refreshes etc. It just sits.

 I assume this is either a bit of code Firefox doesn't like (unlikely since you're using Firefox as I assumed you would ;-) ) or a setup in my Firefox - which is curious because other things work, and javascript clearly works. However, I know that the server setup works with IE....  Puzzled!

 Any suggestions or questions gratefully received!

Thank you.