I never understood the concept of Multi-Account Containers. I would guess it makes sense when one hardly closes the browser (and clears the cache) and has dozens of tabs open? What is a standard use case (home/work)?
I simply use Temporary Containers to isolate tabs (or different domains) and login to websites either by hand or via KeepassXC's browser integration. No per-container setup needed, every tab just lives briefly in its own little container.
1. Further defense in depth against common XSS attacks: if you only ever log into banking sites in a Banking container, then some random ad-sprung malware trying to access your bank in any other container will never find a logged in banking session to hijack (no matter how good your bank thinks they are at XSS protection and auto-timeouts).
2. Isolating tracking cookies from major ad companies. This reduces cross-correlation between/among them. I have separate containers at this point for all of: Google account usage, Twitter account usage, Facebook account usage, and Amazon account usage. The amount of targeted advertising I see decreases every time I sequester one of these major accounts into its own container. The trade-off is the increase in ReCaptchas needed and the increase in advertising companies complaining I must be using an "ad blocker". (I don't have any ad blocker extension installed, just this strategy of isolating major cookies to specific containers and Firefox built-in Enhanced Tracking Protection.)
I simply use Temporary Containers to isolate tabs (or different domains) and login to websites either by hand or via KeepassXC's browser integration. No per-container setup needed, every tab just lives briefly in its own little container.