The log files tell the true story: Webmaster related: 47% use firefox Non Webmaster related: 11% use firefox That's an average on my sites, and I am sure it does vary to some degree for others. There's been a few people that were upset about Firefox plugins blocking their ads so they blocked/redirected Firefox users to a separate page explaining why they can't view the Website. Their arguement is that blocking ads is essentially theft. I'm not going to debate that because I'm sure there are plenty of opinions to support both sides of the issue. However, I will say that if someone is so inclined to install an ad blocker, chances are they won't click many ads anyway.
This is the key point, I think. The people who hate ads that much aren't your market. I have AdBlock installed. I use it selectively to ban ads from networks that have those really jarring flashing brightly-coloured things that make it impossible to read the other content on the page. I wouldn't surf without it. If the cost of that ability is losing 10% of my CPMs, well, blame the people who made ads so annoying that someone felt the need to develop AdBlock in the first place.