You can save current position in displayed webpage using shortcuts Shift-Alt-0‥9;   later, you may return to saved position by pressing Alt -0‥9.   Markers created in this way are kept in browser's persistent memory and they will be restored next time you open the page.

notes

• Before jumping to marker the script stores the current page position, so you can get back by pressing Alt-minus (that on the right of zero.)
• When you open webpage the script tries to restore previously saved position #0
• Positions are calculated in screen's pixels, so if you alter zoom factor, or text column width, or something else, markers may become invalid :(
• You can change script's keyboard shortcuts using GreaseMonkey menu:

  User Script Commands → Y-marker hotkeys

  
  (I use Win key - it seems working on any site.)
• press Alt-Shift-0   twice to set starting Y-position for any page in the whole domain
• click on pop-up message at top-right corner to remove all markers for current webpage.  
• you can control the script through bookmarklets:
    - jump to bookmark #0:

  javascript:postMessage('Y-marker0','*')

    - set up bookmark #0:

  javascript:postMessage('Y-marker0=','*')

16.03.04 * bookmarklets work in Chrome