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.
If you want to view the current HTML source in Firefox you can use Ctrl+A and then right-click > "View Selection Source".
I think it's more a question of what do you expect to see when you "View Source". For example, I have messed around with document.write a lot and it's pretty obvious to me that, if I use view source then it's going to give me the source and any changes done to it my document.write/open/close.
In this case since document.write is used after HTML parsing has been completed it replaces the whole page and thus makes view source rather pointless.
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.