CTRL+TAB = recent [1]

It's a common practice that if you press CTRL+TAB you are switched between the current tab and the most recent one.
Try fx in Firefox

Please implement this feature in XShell 4

Program Ver. : Xshell 4

Thank you for this request.
This feature has been requested before and is already on our queue for development.
We hope to see this feature come alive soon!

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Technical Support