Firefox 35 fixes a highly annoying bug introduced in Firefox 34. It had switched to the silly GNOME mouse scroll behavior. This is different than the behavior on Windows or Mac. A left click would jump to position rather than scroll down one page. A right click would scroll down a page. Windows and Mac scroll down a page with left click.
This was highly annoying. It was firefox bug 803633. The fix is:
Create a $HOME/.themes/Adwaita/gtk-2.0/gtkrc file with the following content:
gtk-primary-button-warps-slider = 0
You need that include because the theme includes setting the warps variable. This changes the order of evaluation to include the theme then change the variable. Without the include you change the variable, then the theme is included undoing that work.