Most notably, Version 40 adds support for Windows 10 devices. A reduced UI gives more space for viewing Web pages and larger, different color icons keep the browser in line with the new Microsoft design scheme.
Desktop and mobile versions use Google’s Safe Browsing service, which identifies potentially threatening malware downloads and warns users about possible security issues. This was already available on desktop builds, but it’s now an option on Android too.
For desktop users, Firefox 40 provides a warning message if you try and access a page that’s a known threat.
Android users get support for the Android Presentation API when screencasting from their device and access to their Web history simply by holding down the forward or backward navigation buttons in the app menu, which is handy for navigating across multiple pages.