I've been wanting to jump into the Mac scene for awhile now, and these new iMacs are exactly the excuse I've been needing to force myself to do it. An iMac with a dual-core Intel chip? Man, two years ago that would have been unthinkable.
The latest version of Firefox is out (mozilla.com) I haven't toyed with any of the betas, but, thus far, it's fairly smooth transition. The Add-On and Extension interface is really improved, and the browser itself markedly faster.