Well that's strange. :/
Maybe you should check the "Don't ask me again" box and then tell it to stop the script. If you haven't already tried that.
I have a minimal knowledge of Macs, sadly. Sorry I can't help more. :P
"Google Ad Services"? Looks like it's an ad causing trouble. You may want to try out Adblock Plus, and see if that helps.
Edit: Link to the Add-On: https://addons.mozilla.org/en-US/firefox/addon/adblock-plus/