Would you mind expanding on that? I'm on MacOS and neither Cmd-. nor Ctrl-. seem to be bound in Firefox 57.b03. Is there a way to list all keybindings that are in effect? Cmd-click on a link does work for me as a way to open a new tab in the same container, which is a possible workaround. Otherwise this is making containers very hard to use for me.
EDIT: Cmd-click on the "New Tab" + icon does open a new tab in the same container.
Do you have the Containers extension installed as well, and the toolbar button enabled? That's what it's bound to. I don't know of a way to list all keybindings.
EDIT: Cmd-click on the "New Tab" + icon does open a new tab in the same container.