Mozilla launches Firefox 59 on Android, here's what's new
The most important seems to be the ability to use Firefox for Android as an Assist App, which will allow users to start a search by long-pressing the home button. In order to take advantage of the new feature, you must head to Settings / Apps & Notifications / Advanced / Default apps / Assist & voice input / Firefox.
Additionally, Mozilla included support for HLS (HTTP Live Streaming) playback for much better compatibility with video sites. As far as other changes go, it appears that the “about” page was removed, so you'll now have to use the “About Firefox” dialog instead. Also, you can now lock video full screen in landscape mode.
The official changelog mentions that Firefox Private Browsing Mode will now remove path information from referrers sent to third parties in order to prevent third-party data leakage. Finally, various security fixes have been implemented too.