FCM allows you to receive notification messages in your web application running in different browsers with the support of service worker. The service worker is a browser script that runs in the background and provides features such as offline data capability, background data syncing, push notifications, and more. The service worker support is available in the following browsers:

• Chrome: 50+
• Firefox: 44+
• Opera Mobile: 37+

Using service workers, people can carry out some malicious activities, such as filtering the responses or hijacking connections. To avoid that, service workers can only be used on pages served over HTTPS. Hence, you will need a valid SSL certificate on your server if you want to use FCM. Note that in the local environment, you don't need SSL; it works at localhost without any issues.