A couple of caveats, though:
* If you disable Multi-Account Containers at ANY TIME, all of your configuration will be gone. So, don't ever disable it. Or just be prepared.
* They don't sync with Firefox Sync.
* I use Alfred and in some of my workflows, when I have URLs that are designed to always open in a particular container, FF just won't open the URL. /shrug
* Always opening URLs depends on the URL. This works great for apps which give you account-specific URLs (e.g. Harvest, Jira, other services...) but doesn't work at all for stuff like Gmail or Drive. I wish there was a way to get FF to ask you how you want to open a URL like that.