I styled my scroll bar using CSS and it is working fine on some browsers like Chrome and Internet Explorer, but not on others (e.g. Firefox). What’s the reason for this?
Content code: HTML
Styling code: CSS
Scroll bar working: 
Scroll bar not working: 
>Solution :
::-webkit-scrollbar is not a standart across the web.
If you wish to make your website usable on every browser you should also add scrollbar-attribute.
Here is an answer to another quite similar question.
But be careful. Support for this might not be perfect as of now.