The Web Reader is now available as a Firefox extension here: Readlang Web Reader – Get this Extension for 🦊 Firefox (en-US)
It works just like the Chrome extension.
Thanks to @Rick_E for requesting this and showing how the Chrome extension could be adapted to work on Firefox.
If you notice any problems with it, please let me know.
(Note: I don’t think the add-on has been subject to human review by Mozilla yet, and if it is in future, there is a small chance that Mozilla decides to remove it. They did this to my original Firefox extension back in 2013, but I think the new version should be acceptable since this time all of the javascript code directly run by the add-on is bundled along with it.)