userChrome.css changes to Firefox only take effect on Ubuntu, not Mac

Question

I have a userChrome.css file that I use to remove unused menu items from Firefox context menus on my Ubuntu machine. Here is the contents:

    :root[inFullscreen] #PersonalToolbar {
    visibility: visible !important;
  }

  .openintabs-menuitem, 
#context-openlinkinusercontext-menu,
#context-savelinktopocket,
#context-pocket,
#context-searchselect, 
#context-sendlinktodevice,
#context-viewpartialsource-selection,
#context-selectall,
#context-sendpagetodevice,
#context-bookmarklink,
#context-savelink,
#context-reload,
#context-bookmarkpage
    {
    display: none !important;
    }

This file successfully removes the listed items from the context menu when running Firefox on Ubuntu.

However, when I paste the same file into the /Users/username/Library/Application Support/Firefox/Profiles/0uhtc3la.default-release-2/chrome folder on my Mac, none of the desired changes take effect.

I have changed toolkit.legacyUserProfileCustomizations.stylesheets value to true in about:config, and have also enabled browser chrome/debugging toolboxes and remote debugging. The only difference I can thing of is that one Firefox instance is running on Mac and the other Linux, although of course I can't be 100% sure that this is the cause. Does anyone know what is causing the discrepancy, and how I can fix the bug on my Mac instance of Firefox?