The second beta fixes a reported 190 major issues.  Most of the issues revolve around responsiveness, crashes, or broken features.  Among other things, the fixes help to improve the thumbnail preview, which is offered when you press Ctrl+Tab.

Speed in the new browser is improved in some areas, but still lags in others, reportedly.  According to BetaNews, CSS and Javascript is faster in 3.6.  However, the browser still lags behind Chrome 3/4 and Safari in the critical onLoad() event, which allows the browser to begin executing code while the remainder of the page downloads and renders.

Reportedly, the 3.6 beta 2 is also slower than Firefox 3.5.4 at standard page loads and loads of pages with HTML tables.  Overall, the browser is more responsive and slightly faster, by feel.

If Firefox 3.6 beta 2 can deliver stability in the long term, Mozilla may have a winner on its hands.  While Chrome, Opera, and Safari have all been faster than Firefox for some time now, many users prefer Firefox due to its interface and easy-to-use extensions.  Still, with Chrome and Opera gaining ground, its important for Firefox to continue to deliver improvements.

You can download the new beta from here and try it for yourself.