> but the official plugin for managing them was awful

I wish we'd stop being so hard on products. It's likely those developers read these comments. Having babies be called ugly by your peers is rough, actively trying to be more respectful on the internet leads to a nicer industry.

EDIT: Just to clarify - I'm not saying your intention was disrespectful. I've written many comments similar to this, and continue to do so, but I've started to try and curb it because I imagine it's what YouTubers feel like reading their comments section.. But this is by our own peers in our industry.

They should not take it personally. The UX/dev people at Firefox should take that as a constructive criticism.

Since when is constructive criticism expressed in words like “awful”?

You should take be asking the question why are the users using words like "awful"

I was quite frustrated by this as well, but then I remembered that the design goal of the extension is allowing logging into the same service with multiple accounts, not isolating services from each other. That is why the secondary functionality, the possibility of isolating tech fiefdoms, is less polished. It's not the main show, it's just a happy side effect.

Containerise and Cookie Autodelete together are indeed a very good addition to uBlock and uMatrix, and if you already have some Regular Expression classification rules written for Cookie Autodelete, setting up Containerise is very quick. It works like a charm even with the Firefox Multi Account Containers extension itself disabled.