Mozilla has a lot of interesting information and statistics about installed plug-ins, and one of my personal favorites to examine is AdBlock Plus by Wladimir Palant. Here is the stats page for it. Looking at a chart over the last 12 months, the number of active installs is fairly stable except for times like Christmas when a lot of people are seemingly not at their computers. Weekly ebbs seem pretty reasonable as well:

Right now there are 10.63 million live installs (of 78 million-odd total), with 3.5 million of those in the US, or 33%, followed by Germany at 2.1mm (19%) and Russia at 1.01mm (10%). France trails a bit further behind with 813k or 8%. The most common OS is Windows but Linux is right up there which clearly also adds to the fact of this being Firefox, to make it a far less mainstream crowd that our Internet Explorer group. It’s geeky.

While these numbers are significant they are not huge (about the size of the 400th biggest US website or so at the 3.5mm mark, if you believe the Quantcast figures) and no immediate looming threat to the advertising market. The Adblock product, on Firefox and with some configuration needed, is simply not a mainstream product. Me wonders though what some of the catalysts would/might be to make this a more mainstream phenomenon.