Blocked it 👍
Two other communities on the list, one from feddit.rocks and one from lemm.ee, seem to be on the list too given that they're also unblockable, and should probably stay that way given that unlike the thunder one, they seem to target specific Lemmy users.
I guess I'll ask the Arctic developer if there's a way to detect instance-blocked communities to filter those two communities from my list as well.
While all websites should have valid certificates, I did run into a case a week ago of the Sears Home Services website seemingly having an invalid one.
In trying to cancel my grandma's account with them, I found that their website wasn't well mobile-optimized, and Firefox refused to load it, so I ended up having to use Chrome to load the website and cancel the service.
Loading such a website should be allowed, perhaps with a warning about phishing and/or other security risks. Given that there are valid websites with invalid certificates, their inaccessibility risks discouraging people from using Firefox.