Chung-chieh Shan noticed that in the photograph I took of the Helvetica and Akzidenz Grotesk comparison, that the legend text for Helvetica is set using Akzidenz Grotesk and the text for Akzidenz Grotesk in Helvetica. In a different comment he pointed out the rather useful website Identifont that will attempt to identify a typeface for you by essentially playing »twenty questions« with different attributes about the glyphs. It took seventeen questions to tell me that my current theme uses Lucida Sans (or two very closely related typefaces). Which I initially thought was wrong, because I could have sworn I had it configured for something like Optima, but I doubled checked the style sheets and it tells me:
font-family: "Lucida Grande", "Lucida Sans Unicode", Verdana, Arial, sans-serif;
So it seems it is doing a reasonable job, though I wasn't able to determine whether it is possible for Firefox to tell me what it is actually using to render the page. MacOS X Fontbook tells me that I do have Lucida Grande installed, so that is probably what it chose.