Add-ons have proven to be a major benefit of Firefox (and Thunderbird). The download counter at addons.nozilla.org (AMO) recently hit the 600-Million mark. Keep in mind the actual number of add-ons downloaded is likely much higher being this figure does not represent those add-ons downloaded from developers’ or third-party sites. The chart on the right breaks down the percentage of these 600-million downloads by application (Firefox 1.x, Firefox 2.x, Firefox 3.x or Thunderbird). I am kinda mixed on the figure for Thunderbird, as I would hope it would be higher but then again 4% is pretty good given the significant…