This addon is deprecated. You should use Containers instead.
https://addons.mozilla.org/en-US/firefox/addon/multi-account-containers/
What is priv8? This is a Firefox addon that uses part of the security model of Firefox OS to create sandboxed tabs. Each sandbox is a completely separated world: it doesn't share cookies, storage, and a lots of other stuff with the rest of Firefox, but just with other tabs from the same sandbox.
Each sandbox has a name and a color, therefore it will be always easy to identify which tab is sandboxed.
Also, these sandboxes are permanent! So, when you open one of them the second time, maybe after a restart, that sandbox will still have the same cookies, same storage, etc - as you left the previous time.
You can also switch between sandboxes using the context menu for the tab.
Here an example: with priv8 you can read your gmail webmail in a tab, and another gmail webmail in another tab at the same time. Still, you can be logged in on Facebook in a tab and not in the others. This is nice!
Moreover, if you are a web developer and you want to test a website using multiple accounts, priv8 gives you the opportunity to have each account in a sandboxed tab. Much easier then have multiple profiles or login and logout manually every time!
Priv8 is released under Mozilla Public License.
Manage your tabs in sandboxes increasing your privacy.