I believe Google may be circumventing cookie blocking currently as it did with Safari a short time ago. I use the blacklist included in cookie options in Firefox, repeatedly, and Google will not stay blocked. Frustrated, I installed CookieKiller (the extension) and it works a little better (Google seems to stay away for several sessions), but Google cookies eventually keep coming back. I do have a lot of extensions installed, but don't have the ability or time to use process of elimination to deduce whether one or a combination of them is at fault. This has gone on for months.
Restart in SafeMode which kills them all & see if you still are getting the cookies, if not then you know its an add-on you have.
(If you're not using it, switch to the Default theme.)
You can open the Firefox 4.0+ Safe Mode by holding the Shift key when you use the Firefox desktop or Start menu shortcut.
Or use the Help menu item and click on the Restart with Add-ons Disabled... menu item while Firefox is running.
Don't select anything right now, just use "'Start in Safe Mode"To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut (without the Shift key) to open it again.If it is good in the Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one.