Neil Lee has updated his optimized Firefox builds for Firefox 3. The architecture-specific versions of Firefox 2 had been dubbed BonEcho, but Firefox 3 brings a new moniker: Minefield. I'm unsure as to the intended implications of the name (it sounds like the perfect way to refer to an alpha release), but I've been running the Intel version with great results.
If you were a user of BonEcho, you were used to the icon being visibly different than the standard Firefox icon. The icon for Minefield, designed by Adam Betts, is a slight variation on the standard icon and not immediately discernible as a deviation from the original. This has confused a few people, but it's definitely a less jarring transition. You still get the title "Minefield" wherever Firefox would have shown up in the interface, so you know what you're running.
As far as performance, I haven't run any solid benchmarks but have noticed what seems like a significant decrease in initial load time and and improvement in general responsiveness when comparing clean installations (no addons) of Minefield to the standard Firefox 3 build.
When I went to grab Minefield, I also discovered a link to the GrApple theme. It's designed to look like Safari and, being a Safari fan, I've fallen in love with it. If you're not overly attached to the default Firefox 3 look (which I didn't mind to begin with), there are great screenshots of the multiple versions available.
Minefield is currently available in G5 and Intel flavors. Neil is also working on a set of instructions for building your own optimized Firefox builds at home for the DIY speed enthusiast.
Clarification: As pointed out in the comments, the name "Minefield" has been used for Mozilla builds for a while. These optimized builds use the codenames so as not to infringe on any Firefox trademarks.