Could you test in a new Firefox profile. In addition to bypassing current add-ons and custom settings, it should have completely fresh settings databases and a fresh cache folder and no Google cookies.

Close Firefox and start up in the Profile Manager as described in Managing profiles. Any time you want to switch profiles, close Firefox and return to this dialog.

Notice any difference?

I'm sorry, I should have mentioned that I tried creating a new Firefox profile as well. When I did this Google ads returned initially, then vanished again when I refreshed or surfed away from a page.

I can't think of any reason for ads to disappear by themselves. I'm confident that Google keeps its AdSense scripts up-to-date for the latest versions of Firefox since there's real money at stake. Strange.

satanism said .....Java is active..., do you mean JavaScript ?

JavaScript is not Java

enable javascript and check it, if you don't have already.

thank you

yes, I mean javascript, sorry.. I was typing too fast. It is, and has been, enabled.

Do you have all cookies enabled ? (Accept third party cookies also?)

Enable and disable cookies that websites use to track your preferences

thanks again