Hi there,
I’ve recently written a module to inject custom code/styles for simple tasks and modifications like this. https://github.com/shin10/MMM-FF-code-injector
The following config for example will show the clock module only Mon - Fri, 06:00 - 10:00. I just tested it with MMM-Pages and it works quite nice.
{
  module: "MMM-FF-code-injector",
  disabled: false,
  config: {
    position: "top",
    scripts: {
      intervals:[  {
        interval: 60 * 1000, // test all 60 seconds
        description: "Show module only Mon - Fri, 06:00 - 10:00",
        func: function() {
          MM.getModules().enumerate((module) => {
            if (module.name === "clock") { // name of the module to show/hide
              let now = new Date();
              let day = now.getDay();
              if (day >= 1 && day <= 5) { // mon -fri
                let hour = now.getHours();
                if(hour >= 6 && hour <= 10) { // 06:00 - 10:00
                  module.show(1000, { lockString: "code-injector"});
                  return
                }
              }
              module.hide(1000, { lockString: "code-injector"});
            }
          });
        }
      }]
    },
  },
},