This topic appears to show up a few times, but none of the solutions work for me (e.g. they were using A/V or "it just worked"). The error I get is also different. This a pretty fresh install (migrating from Chrome so I'm currently re-logging into all my sites to ensure I can access everything I need.. and going through MFA hell heheheheh). Everything is mostly fine, except for Synchrony Bank. I go to login at https://auth.synchronybank.com/account/login, enter _any_ username or password (doesn't matter if I enter the right details or the wrong ones), and as soon as I click "sign in" I get an error page Looking in developer console, the error reason given is NS_ERROR_NET_INTERRUPT while trying to POST to auth.synchronybank.com. Every so often I get PR_IO_TIMEOUT_ERROR but over 90% of the time it's NET_INTERRUPT. This occurs also in private browsing and also in safe mode. I even created a new profile and tested it from there; same issue. I've tried lots of things related to this (e.g. setting network.http.http3.enable to false), added the domain to the advanced tracking exception list, and I'm running out of ideas. I can see traffic flowing to akamai but (it being https) I can't see what's inside the traffic. Could akamai be objecting to something in the content? FWIW, I can login with Chrome on the same machine.