Firefox 53.0 drops pre Pentium 4 and Opteron Linux support. Firefox 53.0 also has support for WebM videos with alpha channel, lightweight theme changes along with new light and dark lightweight/compact theme options shipping, the Reader Mode can now display a time estimate for reading a given web page, and more. Mozilla also decided to remove the Aurora channel from their release cycle. There are also other changes in Firefox 53.0, but mostly affecting macOS and Windows users (like a "Quantum Compositor" being used now by Firefox on Windows).
Firefox 53.0 can be fetched from the usual sources depending upon how you keep your web-browser up to date for your OS.