I have the same problem. I first encountered the pop up 2/22/2012 while visiting coupons.com. I updated to Firefox 10.0.2 plus all add-ons. I am still searching for the source of the problem. If disable add-ons, and restart it does not occur.
I have just a few add-ons , which are the basics.
You have to kill it in two places. FF must be generating revenue from this.
1) Click on the Plug Ins tab.
Disable both "Coupon Inc" addons. For some reason I had two.
2) Click on the Extensions tab
Disable the "Start Now toolbar"
What a pain.