Firefox shortcut to open a particular account container

According to the Multi-Account Container plugin page, you can use Ctrl + . to open the panel. Then simply press the down arrow (or tab) and enter.

Ctrl + . (period) keyboard shortcut is available to open the containers panel. You can then use tab and the enter keys to open new container tabs.

After Ctrl + . you can also use the numeric keys in order to select a container from the list. (Thanks cyberalex)

It doesn't seem to support opening a specific container with a designated shortcut.

As of version 7 of the Containers extension, ctrl+shift+1-9 works without any further add-on. From the release notes:

Set shortcuts in the add-on preferences panel: Ctrl + Shift + {Num}

Which containers correspond to which number can be configured in the add-on preferences.

As mentioned in other answers, ctrl+., tab also works.

On MacOS it's cmd+shift+{num}

I use https://addons.mozilla.org/en-US/firefox/addon/easy-container-shortcuts/

AFAIK, there are no firefox built-in shortcuts