Rated 5 out of 5 stars

I'd like understand why two private tabs know from eachother, which i think shouldn't be, but that's probably a firefox issue:

When i log in to a site that uses cookies, i can use a private tab to log in with another account, but when i open another private tab it's already logged in just like in the other private tab. In my understanding however, a tab (or window) would only be really private if it doesn't transfer any cookies whatsoever especially from other sites (possibly this is already the case).

This review is for a previous version of the add-on (0.1.3). 

Yes, this is a Firefox issue: Private Tab just uses built-in private browsing mechanism (and the same will happens with private windows without Private Tab).

If you want only separate cookies, try Multifox http://br.mozdev.org/multifox/