Mozilla expects to release the final version of Firefox 4 (which has been in beta since early July) before the end of the year. Beta 5 comes out this week. This version will include a new security feature, the HTTP Strict Transport Security (HSTS) standard, which allows web site owners to ensure that the browser goes to the secure version of a web site instead of first going through a non-encrypted HTTP address. You can read more about this and other Firefox 4 features here:
http://itmanagement.earthweb.com/features/article.php/3902446/Firefox-4-Set-to-Improve-Security