for the record: mastodon.social does not block us (berserker.town). neither does several other quite popular instances.
usually, it's smaller instances with admins who are a bit too block happy who do it, in my experience.
one of the first things to do when moving to an instance is to check who they block. tells you a lot about who runs it.
EDIT: and yes, i should have thought to check firefish.social before signing up, except the site is very slow right now. also, i had never heard about them until today, so i figured they'd ever heard about us.