Just Read - Text to Speech ਵਲੋਂ Ash

To read:
Highlight text, right click, select "Just Read" -> "Read aloud".

To stop reading:
Right click, select "Stop reading"

Adjust read speed:
Go to the extension's "Options" tab and adjust the utterance rate.

See source code here: https://github.com/asnewman/just-read-firefox/