I had been having a similar problem.. I'd get a green square box popup about every 4th time I went to expedia.com I finally just removed a bunch of plugins and extensions from firefox and the issue seems to have gone away.

I probably should have removed the plugins/extensions one at a time to see which caused the problem.. but my suspicion was that a jzip extension may have been the culprit.

Uninstall Panda Security Toolbar from Add/Remove Programs.

You have to kill it in two places. FF must be generating revenue from this.

Goto Add-ons

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.

I have found the culprit! It is something that calls itself by the innocuous name of "Browser Companion Helper" . You will find it in tools/add-ons/Extensions. You can either disable it or empty it into the little trash bin by the side of it. And you will not see the annoying coupon tab stuck to your browser again. By the way, this annoying plug could be found in any browser, so look for it wherever you may find it, and get rid of the blasted thing! Good luck, and happy browsing!