I've just downloaded Firefox 3 Beta 4 on my Mac and Windows machines and tried it out. First impression is fantastic but not without a caveat. First of all, all of the Firefox add-ons from version 2.0.0.x don't work at all.
Secondly, Firefox 3 Beta 4 in OS X does some strange rendering. It doesn't render one of my clients'
website correctly in OS X but it looks normal in Windows. If you take a look at the screenshot below, the Contact Us text is rendered just below the cyan box. This doesn't happen on Firefox 2.0.0.x in OS X.
Compare this with the Windows rendering, which is identical to v2.0.0.x rendering.
Other than that, general browsing does feel faster. It uses OS X form controls - which is a bit disadvantage for me because everytime I want to check what it looks like on Windows I'll have to go back to Windows again.