The second alpha release of Fennec benefits from many optimizations that increase performance. Of note is the conditional updating of certain canvas regions, rather than redrawing the whole canvas after each change. This is an oversimplification of the facilitating geektitude, which can be studied over at ars technica.
What is probably of more interest to all of us on a practical level, aside from higher speed, is the integration of Twitter functionality in the address bar. As you'll see from the screencaps below, Fennec has the same basic horizontally-sliding layout I wrote about in an earlier post. What's new is a remaining character count on the right end of the address bar in the last two caps. That's right, you can send a tweet right from the bar with this plug. Pretty cool.
As a side note, Mozilla is working on a Symbian Fennec. So it looks like they're infiltrating the mobile market on nearly all fronts.