Firefox containers vs profiles, how contained are they?

Question

A "privacy by compartmentalization" measure I used to implement in my day to day surfing was to separate each part into Firefox profiles. (Work, Study, Finance...)

Recently I learned about Firefox Containers. It would be way more convenient to use everything in one profile separated by containers.

However my questions are:

1. How strong are these containers? Can a tab in one container in any way know about activity in other containers? Or is that impossible by design?
2. I use some extensions like Cookie Auto-Delete and uBlock Origin. Can these in any way leak data from one container to the other?

And the last thing: it really matters that no leaks happen. If it's 100% the same level of isolation as Firefox profiles, please let me know. And if you can add sources, too, it would be great.

Answer 1

The Multi-Account Containers extension you're referring to is based on the Contextual Identities feature of Firefox. Contextual Identities do not provide full isolation. For example, the history, cached HTTP Strict Transport Security settings and saved passwords are still shared. Some of this is by design, other times it's a limitation of the current implementation. Through the shared data (and due to possible bugs), an attacker might be able to break the isolation. Browser extensions are also not bound by Contextual Identities.

So if you want strong isolation, this isn't the optimal choice. Profiles are better, but then you're still relying on Firefox to separate the data, and it's indeed fairly inconvenient. Consider instead using isolation at the operating-system level like Docker containers or virtual machines.