Hi, you are right, I didn't give you all the information. I have had different MacBooks for about 10 years and, for each one, I configured the key just below the Esc key on the left to allow me to toggle between open Firefox browser tabs - I find this configuration to be very comfortable. But in recent iterations of macOS, it is no longer possible to do this.
A few weeks ago I came across an app called Ukelele which claims it can remap keyboards. I installed it a couple of days ago, opened it, and found it to be a little complex for me and so, before I caused any more damage, uninstalled it.
And that's when my problems started with the f12 volume button.