That would be a middle-click with the mouse scroll wheel. You can check the middlemouse.scrollbarPosition pref on the about:config page.

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

Thanks for the suggestion, but this is set to True and always has been. From MozillaZine:

> Middlemouse.scrollbarPosition > > True: Center the scroll position on the location of the middle click. > (Default for Linux/Unix.) > > False: Do nothing. (Default for all but Linux/Unix.)

This has always worked up until 46.0. Now, it has no effect of you set it to True or False. But I did notice that B1 is now operating the way B2 used to work. It positions the scrollbar at the cursor location rather than scrolling a page at a time.

This looks like something that has been broken in this release. Can anyone else confirm this on a Linux system? I will report it as a bug.

Looks like this is a known bug related to use of the GTK+ 3.0 library. See:

https://bugzilla.mozilla.org/show_bug.cgi?id=1197590

Workaround solution by Jim Rees, from that Bugzilla bug report:

Create/edit: ~/.config/gtk-3.0/settings.ini

Add following to this file:

[Settings] gtk-primary-button-warps-slider = false

Restart Firefox and scrollbar clicking works as it always has.