Comment 19 for bug 1576052

If it's done on purpose, it needs a good reason. And I don't see any good reason to break OS-wide, old, bug-free and cross-platform standard behavior.

Admittedly, I've yet to use the new GNOME Shell, where the default gtk3 scrollbar behavior may possibly be as I described. However, this package is specifically meant for Ubuntu which has retained the old tried-and-true behavior in all gtk3 apps; therefore Firefox should also conform.