Firefox describes its quarantined domains this way:
Firefox version 115 introduced Quarantined Domains to protect user privacy and security when we discover significant security issues presented by malicious actors. This feature allows us to prevent attacks by malicious actors targeting specific domains when we have reason to believe there may be malicious add-ons we have not yet discovered.
However they don’t go on to explain how to get a list of them! Turns out it’s pretty easy, they’re in the about:config:
Open about:config and then search for extensions.quarantinedDomains.list. Copy and paste the link because Firefox doesn’t like linking to about:config (nor does it allow url filters, grrrrrr 😡).