The color and style of your scrollbar is determined by your operating system, not specifically Firefox.

Wesley Branton said

The color and style of your scrollbar is determined by your operating system, not specifically Firefox.

Okay, my operating system is Windows 10, how can I change de scrollbar color?

Microsoft does not appear to have a native way to change the scrollbar colors. The only thing that I was able to find on the subject was this thread on the Microsoft support forum. Even though the original poster of that question says that the solution does not work, other threads on other websites claim that it works fine.

The only other option appears to be turning on Windows 10 high contrast mode, but this has some unwanted appearance effects.

WARNING: Changing the Windows registry values can cause serious damage to your computer if done incorrectly. DO SO AT YOUR OWN RISK!

Wesley Branton said

Microsoft does not appear to have a native way to change the scrollbar colors. The only thing that I was able to find on the subject was this thread on the Microsoft support forum. Even though the original poster of that question says that the solution does not work, other threads on other websites claim that it works fine. The only other option appears to be turning on Windows 10 high contrast mode, but this has some unwanted appearance effects.

WARNING: Changing the Windows registry values can cause serious damage to your computer if done incorrectly. DO SO AT YOUR OWN RISK!

Doesnt change anything, and I restarted my PC after. But thanks anyway.