This is not really accurate information. The methodology is poor compared to other selector tests, and the results are less accurate and less specific. Selector optimization changes based upon the browser. The slowest browsers(ie6) actually select faster with $(tagname.classname) and other more specific selectors. Of course he didn't mention which browsers he tested in or which version of jquery or if it was an average of many runs.

In anycase it is probably more worthwhile to optimize for the slowest browsers that you want to be compatible with, and those browsers are not firefox unless you don't care about like half of the people on the internet.

edit - just to clarify, based upon his results I'm guessing he's only using firefox to run these tests.