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.

I didn't realize that groups were separate from containers until I checked the context menu and came here to ask about it.

Thanks for letting us know the difference - hopefully the difference includes other benefits such as only having to reload a container, and not all the containers if there is a crash or other failed state.

Groups are joined together in each window. Containers are not.

Lastly, each tab group can contain tabs from different containers - this means they literally just share a group name - everything else can be different, since that's how containers work. The group joins tabs together no matter which container the tab belongs to.