As tab mix plus no longer works, this is certainly a welcome app. It does what it says, it transfers focus to the tab on the left when closing your current tab - But! - it does not prevent the tab on the right from loading, as tab mix plus did! So, even if not focused, your youtube video, or whatever, will play merrily away, even if you are not focusing on it.

If the developers can get Firefox to 'not load' the tab on the right of the closed tab, before it quickly focuses on the tab to the left, then this app will be perfect!