I installed the firefox adblocker plus plugin to see what my website looks like when I block ads. In IE and Chrome the ads disappear (as would be normal with an adblocker) but in Firefox a sharebutton for stumbleupon appears where my adsense block used to be.
This is very strange since I have no sharebuttons on this page. When I disable the adblocker the stumbleupon button disappears and the adsense ad appears normally.

Anybody got an idea what happened? This is really driving me crazy!
There is no code whatsoever from stumbleupon on this page and still the button is there. I can't get rid of it.