Based on a lifetime of using web browsers, I expect that when a browser has an active throbber, it's downloading a page. I've never seen multiple throbbers, though, and I just noticed that Chrome has (at least) two.
Out of five open tabs, three were static and just sitting open in the background, so they displayed favicons as expected. The other two were loading, but one had a slow counter-clockwise throbber while the other had a fast clockwise throbber. What's the difference between the two (and any others that may exist)?
Firefox 4 has been released, and it has the same behavior (or at least a similar-looking behavior). I still haven't found documentation.