[Schlaefer]

I moved on to sideberry and hide the normal tab bar completely. But if you open two identical tabs in the default layout I have no clue which one is active anymore. You can't understand that UI, you have learn it. It's infuriating.

[SiempreViernes]

It doesn't render the one you clicked on in a different colour? That might be a issue with the theme you use, my firefox shows the active tab in a clearly different colour from the inactive one.

[Schlaefer]

It changes the color, but that color has no inherent meaning. Let's eradicate any prior knowledge of what was clicked. Let's say you make screenshot of the current state, and show that to a random person on the street, who has never used FF before.

What are the odds of that person identifying what is currently active?

If the counter is "you know because you remembered" or you "know because you learned" then any of these answers indicate a inferior and non-intuitive UI design.