Seems like it's intended to stop things like data collection on banking sites, and it can be overridden. Very little documentation on it though, so we'll have to see more in practice.

There is however a way of reverting this (for now?):

1. Load about:config in the Firefox address bar.

2. Click Accept the Risk and Continue, if the prompt appears.

3. Search for extensions.quarantinedDomains.enabled.

4. Set it to FALSE.

5. restart Firefox.

Courtesy of pebcak at forums.endeavouros.com

It’s just big tech trying to get it greedy fingers on money anyway they can! I understand it costs money to run websites, but it’s getting to the point where a lot of sites and ad revenue setups are starting to have more ads then actual content, like back in the 80’s and 90’s. And do t forget about the scripts that track everything up do so they can pinpoint ads that will peel to you, at so they think, based on browsing and search history. Hell even brave is starting to put ads and trackers in its browser