Did you look at the privacytests.org link I shared earlier?
It would be difficult to argue, on balance, that Firefox’s privacy protections are better than Brave’s for most common browser privacy concerns. The eBay login/logout thing is just because the cookie for the site gets deleted when the ephemeral container goes away – if it’s more important to you that every time you visit ‘Site X’ that you appear to be a new endpoint, you will need a lot more protections than either (or likely any, aside from Tor) is going to provide. The linked site is more geared towards protecting your ‘identity’ (using that term loosely) on one site against another which I think is your other question – and this is probably the most common definition – and Brave does do very well here.
You might also want to look at the “Sites that clear cookies when you close them” setting in brave://settings/cookies – I haven’t tried this, but it might partially fulfill this for you, although granted it’s a per-site approach. Some discussion here: Brave clearing cookies when I close tabs - #8 by TheWayOfYahweh
Beyond that – if you have ‘niche’ requirements that you cannot live without, if you don’t find someone to pay for answering more questions, that are more valuable to you than the other factors, and that Firefox solves for you, just use Firefox.