Firefox 34 just went to the F20 stable updates. The bookmark sidebar scrolling behavior has changed and I can't find any reference to it, though I probably just don't know what to search for. You used to be able to middle-click on the scrollbar to move it to an arbitrary position, or left-click on it to move it up/down one page at a time. Now left-click moves the scrollbar to an arbitrary position (same as the old middle-click), and middle-click does nothing. Is there some way in FF34 to scroll up and down the bookmark sidebar one page at a time?