Based on the small amount of testing I did on couple versions of Firefox, disallowing third party cookies means only websites you are directly visiting can read or write any cookies.
So if you go to facebook.com and it sets some cookies and later you go to somerandomblog.com that has some images from facebook.com, Firefox will not send cookies to facebook.com, since you are not visiting it directly.
Now obviously if somerandomblog.com has javascript from facebook.com on it, then that javascript can read cookies from somerandomblog.com and do pretty much anything it wants with that page.
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Based on the small amount of testing I did on couple versions of Firefox, disallowing third party cookies means only websites you are directly visiting can read or write any cookies.
So if you go to facebook.com and it sets some cookies and later you go to somerandomblog.com that has some images from facebook.com, Firefox will not send cookies to facebook.com, since you are not visiting it directly.
Now obviously if somerandomblog.com has javascript from facebook.com on it, then that javascript can read cookies from somerandomblog.com and do pretty much anything it wants with that page.