It was the final version to support legacy plugins like Silverlight and Java (excluding Adobe Flash), which were being phased out in favor of HTML5. The Quantum Leap:
This version is historically significant because it was the before Mozilla pushed its sweeping "WebExtensions-only" policy (Firefox 57+, codenamed "Quantum"). It was also the final version of Firefox to support older operating systems , including: firefox version 52.9. 0 esr download