XUL-, RDF- and London-based Joost opened up to the public this week with Windows and OS X versions of their slick IPTV application. See also Miro.
Cross-platform XUL-based web audio browser/manager/player Songbird has made lots of interface improvements in the recent nightly builds (there are recommended nightly versions to try, from about a week ago).
Thunderbird's got funding and a new home, which is good as I've moved to Thunderbird for email on all platforms.
EMusic's XUL-based download manager, EMusic Remote, works well.
A new release of WebRunner lets you run web pages as separate applications. You can use this to run applications like Google Reader, Gmail or BBC Radio Player independent of Firefox.
The Open Komodo Project has released a free XUL-based code editor, Komodo Edit (good but lacks auto-clean up of indentation), and plans to produce Snapdragon, a web development tool integrated with Firefox.
And of course Firefox is getting better all the time, with a solidifying set of essential extensions - Adblock Plus, Download Statusbar, DownThemAll!, Firebug, Greasemonkey, Growl Notifications, It's All Text!, OpenSearchFox, Secure Login, Slogger and Stylish.