Firefox 21, per window private browsing not working. The cookies are still shared among private browsing windows and tabs. Even with private tab addon, still not working. If I log into google in one tab, I am still logged in with that account in new private tabs and private windows.

All private tabs and private windows share the same cookies.
So you have two sets: one for the normal window and one for Private Browsing mode tabs and windows.

Then what is the point of per window private browsing? or what does it even mean? Simply not caching to disk?

It means the same as before: no data from a private tab is stored on disk.
Only previously you had to close the normal window or at least could only use private mode or normal mode and not both at the same time.
Now you can have both open at the same time and switch from a private tab to a normal tab.

Thanks. So per windows/tab cookie(session) is still not possible?

No. All tabs of the same type share cookies.

One more question about cookies in private browsing mode. I don't like browse a web using usual mode 'cause I don't like tonns of sites in a history. But when I start with private mode every time I get empty set of cookies and this was not quite good for me 'cause some sites store its options needed for me. In a prev. versions of FF I solved this issue in such a way: I stored some cookies in a file and every time I started private browsing I loaded them from file. But now this method gives nothing: in cookies manager all set of "non private" cookies is shown both before importing and after importing them from a file and I need to reconfigure my sites every time I start private browsing. I'm sorry for my English.