I’ve been using the Deer Park alpha (Firefox 1.1 alpha) for a few days now. So far, it’s been very stable and very fast. It also includes a couple of regular annoyances with Firefox 1.0… those things that don’t really stop you from doing what you want, but annoy you every time you hit it (which is frequently).
When you open a link from an external program (like Mail), it now opens in a new tab rather than replacing the contents of your topmost browser window.
You can middle-click to open a link in a new tab, which something I do *a lot*. Opening a link in a new tab is part of my normal reading flow with Zesty News.
I’d highly recommend Deer Park for other Mac users. The only drawback I’ve seen is that some extensions have not yet been updated for Deer Park. Don’t worry, though… the Web Developer extension is there!