Firefox's new site isolation feature

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I just enabled it and have not noticed any difference. Some things that I thought it would impact but has not:

1. I have 3 tabs open, each pointing to a different area of a forum. The forum will add a count of alerts to the tab name, and it shows up on all three, and is cleared on all when the alerts have been viewed from one tab. I thought that with isolation that this info would be localized to the current tab.

2. Similarly, I have two tabs open, one for the general account and another for a discussion area of that account. If I am not signed into the account then the section at the top right of the discussion tab shows "Sign-in". When I sign in on the account page and then do a refresh on the discussion tab it now shows my userID instead of "sign-in".

Anyone have a sample or a link to what it does or prevents?
It's isolating all sites from the same domain (with special exceptions for things like GitHub) into the same process/access space. That means multiple tabs sitting on the same site are all able to work together. It means that sites referenced from the site, or plugins like LastPass, should remain isolated from other sites. Even sites isolated in a tab are not isolated from the site's cookies (same domain) so you shouldn't expect logins on one tab to not reflect on another tab from the same site--doing it any other way would break the way the web works.
I've notice issues with youtube. Videos are slow to start. Not sure if it from having fission active or not.
Is this feature enabled by default in 89.0?

If not how do I enable the feature?