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Mozilla's Push API gives web applications the ability to receive messages pushed to them from a server, whether or not the web app is in the foreground, or even currently loaded, on a user agent. Firefox and Chrome allow any number of Push messages that generate notifications. Firefox allows a limited number of push messages to be sent to an application that don't display a notification. Chrome versions earlier than 52 require you to set up a project on Google Cloud Messaging to send push messages, and use the associated project number and API key.