Since a couple of years I used on my website a Nedstat webcounter.
It worked perfectly.
Recently this Nedstat company has been transferred to Webstats4u.com.
This transfer has resulted in other 'terms of use' allowing tracking, analyzing and use op pop-up's.
Since 3 weeks visitors of my site complain about popping up ad's about car's, cactus liquids for helping slimming down their weight, etc. etc.
Apparently the technique Webstats4u is able to trick pop-up blockers and also the pop-up blocker in Firefox
Also the Spybotsd immunizer and Spywareblaster immunizer do not catch this splash thingy originating from webstats4.com
Of course I removed this statscounter from my site immediately.
Maybe webstats4.com could be a candidate for the immunization list ?
p.s.: the pop-up appears 1 of 5 visits from a visitor. So it is more or less irregular.