According to RapidRelease/Caleander on the Mozilla Wiki site, Firefox 53 scheduled to release on April 18, 2017,  but it has changed now due to Easter falling on April 17 in some countries, hence the company has decided to move Firefox 53 merge and release dates by one day. This means merge will happen on 18th and release goes on Wednesday, April 19th.

Below links reveal more information on upcoming Firefox.

Firefox 53 adds two new Compact themes

Firefox 53 shows reading time in Reader Mode

Firefox 53: Shows Notification about deprecated plugins