macOS

This setting can be found by going to System Preferences → Notifications → Do Not Disturb.

Uncheck the box with the "From" and "To" times.

 

In the same place as the Do Not Disturb setting, look for your browser in the list.

Make sure the alert style is set to either "Alerts" or "Banners".

 

 

Once you have your settings correct, you can test to make sure notifications are coming through with the browser notification tester.