Mozilla 1.8 Alpha 5 has been released, after a short freeze period. This latest alpha version of the Mozilla Application Suite features around 600 bug fixes. The Mozilla 1.8a5 Release Notes have more details; there's no What's New section yet but a rough changelog of 1.8a5 fixes is available.
Mozilla 1.8 Alpha5 README
Download Mozilla 1.8 Alpha 5
Mozilla 1.8 Alpha5 README
Download Mozilla 1.8 Alpha 5