I would rate it 10 over 5. The add-on says what it does and does what it says. It may seem obvious but I'd like to suggest the developer to highlight that if a bookmark adheres to the "ext+container:name=MyContainer&url=" syntax it will open in "MyContainer" directly. That's a very good thing to have

Does what it says and does it right.
I love this.

Works well with bookmarks! Thanks!

I have been waiting for something like this for a long time. I can finally set up my password manager to open different accounts on the same site in different containers.

Edit 09 May 2020
Here is an example you can share. I use KeePass 2 password manager, which allows an override URL for specific entries. I have two Amazon accounts, which I open in different containers. In the Override URL field of both, I put
cmd://{FIREFOX} ext+container:name={GROUP}&url="{URL}"

The entry I put in KeePass' Work folder automatically expands to
"C:\Program Files\Mozilla Firefox\firefox.exe" ext+container:name=Work&url=https://smile.amazon.com/

The entry I put in KeePass' Personal folder expands to
"C:\Program Files\Mozilla Firefox\firefox.exe" ext+container:name=Personal&url=https://smile.amazon.com/

Exceptionally useful.