Today I changed the ads on one of my pages. To make a long story short, BOTH MS Internet Explorer and Apple Safari display the exact same 4 ads in a 336x280. Very well targeted. Over and over again. But Firefox shows the same 2 DIFFERENT ads in the 336x280. Over and over again. Also, very well targeted. Sooooo, how come? Is it a cookies thing? I'm sure somebody can shed some light on this. I'm going to go and delete Firefox cookies now.
Yup, it's kind of normal. It's the same thing that happens when you refresh a page -- the ads will, in all likelihood, change. Adsense cycles through different ads with every refresh, so I'm assuming it works likewise if you have three different browsers (of different types or the same) opened up to the same page.
By "over and over again", I meant refresh after refresh after refresh. I burned up a lot of impressions. I left. I came back from a different angle (link from other pages). Always the same. Anyway, I dumped the Google cookies, and the Forefox ads INSTANTLY changed to the exact same 4 ads as displayed by IE and Safari, in the same order. So, I am guessing that cookies may be involved. The thing is, I kind of liked those other two ads. Ha-ha-ha. Be careful what you wish for - you may get it! I'm sure, as you say, they will cycle back in again, as well as others. Kind of interesting, though.