If your referring to something like Chrome where you can have multiple instances of Gmail opened. Then it's not like that I think it switches the account logging out the prior to switch to the next account. Only way to know is if you did the test yourself since your asking it shouldn't be that hard for you to do it yourself to see if it opens tabs for same yahoo email with different accounts opened as example.

No, I am talking about container as per this extension that provides better security and privacy isolation than just "tab groups" if they are just "visually organisational" (sadly it looks like they are)

https://support.mozilla.org/en-US/kb/how-use-firefox-containers

It's not enabled yet in my firefox (progressive rollout) so no I couldn't test.

Tab groups are made for organizing your tabs, they aren't isolated like containers. These features are independent of each other.