Thanks for the feedback.

(1) Keyboard shortcut --> webextensions on Firefox have very limited choices for keyboard shortcuts, many of which are already used for standard browser functionality; and are not user customizable like in chrome; so I've had to give up the keyboard shortcut after porting my extension to use the new webextensions API on Firefox

(2) Text size --> go to about:addons and click on the extension. There are several preferences/options that can be customized by the user.

(3) Images and videos --> I try not to worry about them much in my extension because my focus is only on reading text. Other extensions surely do a better job and I don't plan on competing with them on this front. You may want to try the built in reader in Firefox (just click on the reader icon within the address bar).

(4) Porting "Just Read" to Firefox --> you should definitely contact the author. Mozilla has made it very easy to port Chrome webextensions to Firefox with minimal changes. There are several help pages and guides.

There is also a Firefox extension that will allow you to directly install a chrome extension from the chrome store: https://addons.mozilla.org/en-US/firefox/addon/chrome-store-foxified/

You may want to experiment trying to install "Just Read" on Firefox yourself using the above extension without waiting for the author to port to Firefox.

All the best in your search for finding an extension that meets your needs.