Interesting attack vector I haven't thought about which could leak information out of a network-locked Firefox Container. It would be under an assumption you have either:
1. A malicious extension installed (you have a much worse problem in this case)
2. A side-effect of an existing extension that leaks information to the outside world. (e.g. translate a part of a page, lookup a word in a dictionary, pre-fetch some images...)
> Firefox's containers are only useful if you want multiple logins
I think there are valid use cases for both Containers and Profiles. You can go down the list to have more and more isolation as needed:
- Grouping tabs to stay organized, no isolation
- Firefox containers, same browser window, shared history & extensions
- Chrome profiles, almost complete isolation within same browser (different processes)
- Separate browser instances
- Separate devices