so i've been using firebird for most of today, and i must say it's pretty cool.
i'm a big fan of the mozilla project in general, if only because XUL is damn cool technology, and i very much like the idea of a lightweight cross platform browser.
i was worried at first (back when they weren't releasing os x builds of phoenix), but it looks like they're well on the way to making firebird into that browser. so far, the only real issues i've found are that startup is a little slow (faster than mozilla, but slower than safari), and there are a number of little annoyances (no hotkey for 'go back a page', open-apple-click doesn't open a link in a new tab, open-apple-h opens the history sidebar instead of hiding the app).
all of these are issues, but they're really to be expected at this point in it's development, and i'm certain that as time goes on it will mature into a fantasticly usable application, both on the mac and elsewhere.