Fun! Simple! Yay!

Wow, also works on Firefox Nightly Android.

на ютубе и некоторых других сайтах не работает

Заявленная функция работает и интуитивно легчайше понятно, как включается.

Easy to use and works well.

Wow I love it

Настройки:
Scrollbar width - Platform Default
Scrollbar thumb color - #1d89f2af
Scrollbar track color - #a69e3128

Жаль что нельзя сделать полосу чуточку шире как в Google Chrome.

This is a primary extension (choose Use Custom Colors) which I think should be a default part of FF.

And changing widget.non-native-theme.scrollbar.style to 4 (from 0) in about:config (and hit the Save checkmark) makes it wider.

And I read with disappointment about the response to the developer by FF to enable wider widths:

Rather, as Custom Scrollbars beatifically examples, customization enhances browsers. Thank God.

(Note that Custom Scrollbars will not work in Firefox system pages as this).

A great Add On I sadly have to leave behind because Firefox 100 finally changed the scroll bar in Windows 11 to the version I always wanted, it will still be used for my Windows 10 machines, but it was always complicated to be unable to see what point of the page I reached, because I preferred the hidden setting.

Fantastic!
Thanks!

Easy to use, works perfectly, no more ugly grey scrollbar.