The major changes to HTML and the DOM in Firefox 3 are related to security improvements and standards compliance. The Firefox team fixed a security hole that allowed frames and iframes to inherit the parent document's character set. They also addressed vulnerabilities in chrome access by remote hosts and file uploading. New features from the HTML 5 specification are also present in Firefox 3 such as improved drag-and-drop support and new DOM attributes related to focus management.
Firefox 3 extends the canvass element beyond the official WHATWG specification. Namely, it now provides facilities for drawing text on a canvass surface. Firefox 3 also now supports transformation functions for the canvass objects.
There are additional features in Firefox 3 RC1, include animated PNG (APNG) support. Further details are available at the Mozilla Developer Center. Firefox 3 RC1 can be downloaded from the project website.