Many websites nowadays try to force their own scrollbars, like this:

Such custom scrollbars are usually broken in a variety of ways: bugged to the point of making scrolling impossible, sluggish, non accessible, ugly. So I want to force all websites to use the standard/correct scrollbars, which for me in Firefox 39 on Fedora 21 with Plasma/KDE 4.14.9 are: 
How to do so in an efficient way? I don't want to customise every single website with one of those CSS extensions.