Not sure where you get that information from
Probably from playing it themself. Nowhere in the text you linked is stated that you need the client. Only a valid account and internet.
So no, Overwatch 2 on Steam doesn't require a secondary launcher. Source: me, I play it on Steam.
Without having checked the addon you mentioned, it seems like that the author didn't register openai.com as site in his addon metadata. That's why Firefox tells you it can't access the current site (chat.openai.com).
The new domain restriction only applies to Mozillas own domains (mozilla.com), e.g. to prevent plugins to spoof official sites like the addon marketplace.
Just want to leave this here: Brave is a Corporation, which is profit orientated and Mozilla is a Non-Profit-Organisation.
What happened with "clash-verge"? It's archived and the last commit nuked the repository.