macOS
Turn off Do Not Disturb
This setting can be found by going to System Preferences → Notifications → Do Not Disturb.
Uncheck the box with the "From" and "To" times.
Make sure notifications are enabled for your browser
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.