How to : Disable plank icon background (focus)

Hello, i searched a long time without finding.

For example when i start firefox there is that orange "shadow" behind it. And i want to disable it...

But i don't know if it's a Plank feature or Linux Mint feature (os : linux mint 19 cinnamon) because i have this colored shadow with others applications like "Docky" or "Cairo". Where we can disable it ? I think it's an important par of customisation.

Thanks !