User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-GB; rv:1.9) Gecko/2008061004 Firefox/3.0 Build Identifier: version 3.0a2pre (2008061703) If you add the Mailview box to the toolbar - then it allows a switch between Unread and All; but doesn't allow for Threads-with-Unread, which (IMHO) is the more useful option when you are reading by threads so that when seeing a new message I have the choice of reviewing the background to it. This is on the View->Threads sub-menu. Reproducible: Always Steps to Reproduce: 1. 2. 3.