From MozillaZine Knowledge Base
No released Mozilla product currently passes the Acid2 test. However, development versions of Gecko 1.9, which Firefox 3 will be based on, do pass the test.
While Acid2 is a measure of standards support, a browser that passes Acid2 is not automatically more standards-compliant than one that does not. For a more in-depth comparison of standards support, see Web browser standards support summary at Webdevout.