Here’s how I’ve set up F3 to toggle between the last two active tabs in Chrome and Safari.

First, a demo:

Step 1

Download the macro set from https://forum.keyboardmaestro.com/t/macro-set-toggle-between-browser-tabs/6824.

Step 2

Open the file and import the “Browsers” group to Keyboard Maestro.

Enable the macro group and both the macros inside.

Step 3

Select the “@Browser @Toggle to @Previous @Tab” macro and set F3 as the trigger.