I downloaded and installed the most recent Mozilla 1.3 alpha build today (actually from Dec. 12).
For the past few weeks, I’d been using a nightly build I downloaded back in early November. It was buggy, but without assurance that any given night’s build would be any more or less stable than what I already had, I stuck with the familiar.

Initial impressions: The spam filtering still isn’t complete but it works (it just won’t act on the spam it finds–yet). The speed is comparable to anything else I’ve used, and one annoying bug in the mail client is gone. I’ve grown so used to having the spam filtering that I’ll put up with almost anything in order to have it–I get an unbearable amount of spam, and Mozilla quickly identifies it all for me. After a couple of months of using it, I think it’s pretty safe to say only one or two messages per week get past it anymore. I can definitely live with that.

Once when I visited news.google.com in the browser and clicked on a link, after I hit the back button I got a confusing “The file / can’t be found”. The nightly builds I used previously had the same bug. So far that’s the only one I’ve found, and the workaround is to visit a couple other sites, then go back to the troublesome one.

I’ve only been messing with it for a few hours so I can’t make any sound judgments on its quality. But as an evolutionary, not revolutionary, upgrade from its predecessor, it ought to be fairly stable.

If you’re desperate to get unburied from beneath an avalanche of spam and you’re willing to put up with a few quirks from your Web browser in order to do it, this is the most effective filter I’ve found yet.