I do use that extension, but it specifically blocks you from adding new domains to the Facebook container, which is built into Firefox. I can't figure out how to turn off that built in one and use the multi-account one instead.
> I can't figure out how to turn off that built in one
I don't see the facebook container to be built-in. I have only the multi-account container extension installed, and I can make any website to open by default in the container I have created for facebook.
I think, you will have to uninstall/disable the facebook-container extension, and just have the multi-account container extension enabled.
