I have been using the Nightly browser for some time now, and I have found it to be very stable (no significant crashes anyway). As such, I am very pleased with Mozilla's ability to keep their most unstable browser working 98% of the time. Since there are no user interface changes at this time, I am going to give a quick rundown on my feelings for the nightly browser, and what a HTML5 test has told me.
It has more support for the HTML5 specification--I'm just tested Firefox 4.0 on the http://html5test.com website, and got 240 +11 bonus points which is the same for Firefox 5.0a2 and 6.0a1.
This is better than Firefox 3.6, which obtained a score of 155 +4 bonus points.
However, even that is far beyond IE 8, which got a score of 32 +0 bonus points. I am not going to test anything before IE 8 due to the poor performance of IE 8. I also cannot test IE9 since I do not have a copy of Vista that I can use.
On the other hand, Chromium got a score of 273 +13 bonus points--which is higher than the latest Firefox's score by 33 +2 bonus points. Therefore, overall Chromium is the best choice for HTML5 support.
However, none of the browsers have the extensive environment for addons like Firefox, and that is where Firefox shines. Also, there is now no significant difference between the speeds of the browsers to the average person, and now it is features and not speed that browser vendors will have to compete for.