Excellent idea, besides for security/tracking reasons it is very useful to efficiently manage multiple often-conflicting accounts that get on your way when you work (Google accounts, for example).
I would improve the following:
- possibility to reorder the containers
- instead of (or in addition to) long pressing on the '+' it could be nice to have the "top 3" containers inlined as '+' buttons to open a new tab
- perhaps custom icons for the containers? If it works well