The link in my post above got a parenthesis appended to it that doesn't belong there; the URL should be:

https://support.mozilla.org/en-US/questions/1274655

The CSS formatting got messed up too; it should be:

   toolbarbutton[class^="scrollbutton"] {
      display: none !important;
      }

Try this code in userChrome.css.

toolbarbutton#scrollbutton-up, toolbarbutton#scrollbutton-down {display:none !important;}

That worked! But … why? It seems like the new code ought to do exactly the same thing as the old code, so why does only the new code work? Just curious. Thanks in any case for your help!

Your code uses a class name and this classname is no longer present and instead they have given each scroll button their own ID. You can verify this with the Browser Toolbox.

• https://developer.mozilla.org/en-US/Tools/Browser_Toolbox

Oh duh, somehow I can never keep . and # straight. So this works too:

   toolbarbutton[id^="scrollbutton"] {
       display: none !important;
       }

Thanks again – especially for the Browser Toolbox link!