The Mozilla Project issued an emergency release of Firefox 3.0 today. Firefox 3.0.10 fixes a critical security issue introduced by one of the security fixes provided in Firefox 3.0.9. This new issue caused memory corruption that resulted in frequent crashes, especially for users of the HTML Validator add-on.

You can find more detailed information regarding the nsTextFrame::ClearTextRun() bug, bug 489647, on Mozilla's bug tracking website.