They put the details here: https://3g2upl4pq6kufc4m.onion (looks like it only works on the "Tor Browser updated" page, though. You can find the button in the location bar by clicking the icon to the left of the address, or in `about: tor`
Yeah, I had to look for it too. It's in the firefox hamburger menu, where you'll find the add-ons, preferences, etc.
If you click the green onion next to the site URL, you're able to view it too. You can create a new site identity from that too.
There's also a blue button if you click the i to the left of the current address.
Sometimes there's a problem with the identity which hinders the use of all sites. The solution was usually "new identity" or Tor restart. The first one is a little more accessible.
That's only "New circuit for this site", which is different from "New Identity".