perfect addon, would have written it myself if it didn't exist already.

that said I have one idea to increase usability even more: automatically create containers based on the domain
so basically a second mode where containers are created when you open a new tab and it's a new domain, or assigned to an existing container if one exists for this domain (including subdomains).
might be a bit problematic with redirects maybe, but it frees the user from the need to check the domain before deciding if it can be opened in the same or a new container.

just something to think about. good work in any case!