Microsoft has released the Service Pack 2 for Expression Web 3, that provides latest updates to product. This service pack includes general improvements to the application and to SuperPreview.
It includes SuperPreview support for Firefox 3.6 along with improved stability when working with CSS, PHP, and PDF files. The service pack is cumulative update package and can be installed either you have Expression Web 3 RTM or Expression Web 3 with Service Pack 1.
You can get a more complete description of Expression Web 3 SP2, including a list of issues that were fixed, in the Microsoft Knowledge Base article Description of Expression Web 3 Service Pack 2.