I recommended Deer Park a few weeks ago, and now the official Firefox 1.5 Beta 1 is out! A word of warning, however: the Web Developer extension has not yet been updated for 1.5. And, for me, after I ran 1.5 on my Mac, my Deer Park alpha no longer sees extensions. So, I’m basically left without Web Developer until it’s updated. I’m sure that will be soon, but this is something to keep in mind if you’re a heavy user of Web Developer.
Update: the Web Developer extension has been updated to support Firefox 1.5 beta.