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 lower number of add-ons available and the difficulties some users may have trying to install an add-on in Thunderbird. Some other AMO statistics:
News Source: CyberNet News
The Firefox Extension Guru's Blog 
There are some great add-ons out there. I have been trying to keep up with the “good” ones, but there are so many “good” ones that it’s nearly impossible!
[…] Firefox Extension Guru’s Blog points out that over 600 million Firefox add-ons have been downloaded, and that in anticipation of the release of Firefox 3, addons.mozilla.org is being […]
Thanks for sharing this
[…] today has yet to end (though Steve Ballmer thinks it has). Big news for Add-ons this month with 600-Million Add-Ons Downloaded. A sneak preview of the new AMO 3.0 gave us a glimpse of the new and improved AMO engine. Also came […]
[…] use to Firefox automagically installing the add-ons right from AMO, it is a pain. Consider of the 600-Million Add-Ons Downloaded only 2.4 million (4%) were for Thunderbird. The current version of Thunderbird does have a way to […]