The current release of Firefox (version 49) contains an exciting new feature: Narrate.
Narrate is a text-to-speech feature which can be enabled in the Reader Mode view.
Here are a few more details, courtesy of Engadget:
Firefox’s new text-to-speech feature helpfully narrates articles so you can step away from the screen or swap to another tab and listen at your leisure. Reader mode is also getting some additional customization options that allow users to tweak the text, font size or reader voice, as well as new light and dark themes for daylight or nighttime reading that’s easier on the eyes.
We are excited about the Narrate feature for accessibility reasons. A text-to-speech feature built into a free browser is a victory for those who want to see good assistive technology become more widely available.