incrediBILL - 8:34 pm on Apr 27, 2010 (gmt 0)
Let's get back to the FACTs and stop conjecture just for a moment:
FACT - Analytics for my site (and others) says MSIE visitors have a much higher CTR than other browsers
FACT - AdBlock software downloads are statistically insignificant to seriously impact FireFox CTR. Martinibuster pointed out Adblock's download numbers aren't that large compared for FF as a whole.
FACT - AdBlock isn't used on Safari or Chrome which had similar CTR numbers.
FACT - EU has recently started forcing MS to offer new browsers as options and people are adopting other browsers at a more rapid rate.
FACT - Simple math showed that based on the same eCPC that the changing browser demographics caused a decline in revenue.
Moving forward, it's possible that as more potential MSIE users migrate to other browsers, maybe not of their choice even, that their current CTR trend will start to raise the average of the other browsers.
Many have speculated that it's more savvy netizens using the alternative browsers at the moment but I think that trend will change. With both the EU and Google pushing other browsers heavily it's possible that MSIE users switching will cause an upturn other browsers.
However, on the flip side, all the mobile devices using Webkit (Safari,Chrome) like the iPhone, iPad (1 million in the field in less than a month), Android, etc. that the demographics have no choice but the change. I think you'll see less CTR from Safari and Chrome in the long run because of their high volume usage in PDAs and the ads are often off then screen, which won't be as problematic with the iPad.
IMO, mobile will be the biggest downfall for traditional AdSense sites, especially the new mobile devices using a regular browser unless you redesign your site specifically to address the mobile devices.
Still, Windows netbooks are flying off the shelves at record paces (just got one myself!) and that still doesn't explain Firefox.
Perhaps we'll never be able to explain it fully at this time because there are so many different market forces at work and AdSense is complex, but based solely on browser demographics and performance alone, I see the AdSense CTRs we've come to expect hanging perilously on the edge of a serious slippage.