Container Tab Flow by Maxim Baz

This extension allows you to stay within the current container when closing a tab. When the current tab is closed, the tab to activate will be chosen by following these rules:

1. If there's a tab to the right within the current container, activate it.
2. Otherwise, if there's a tab to the left within the current container, activate it.
3. Otherwise, the built-in Firefox logic kicks in, which by default works as following:
a. If there is a tab to the right, activate it.
b. Otherwise, if there is a tab to the left, activate it.