Firefox 49 has been released for Windows, Mac and Linux platforms and available for download. This version expands multi-process (e10s) support by allowing a set of add-ons to be installed that are compatible and whitelisted by Mozilla. And Reader Mode has been improved, Narrate feature has been added it. It reads the content of a page loud. Apart from that, controls have been added to the feature, which lets you adjust fonts, text and voice as well.
Enabling Night Mode for Reader in Firefox 49
You can enable Night Mode for Reader View as well by changing the theme to dark from light, for that, visit a readable page in Firefox browser, click on Reader View icon and click on Aa and select Dark.
From now on, whenever view an article in Firefox browser in Reader Mode, it will be displayed with dark theme.