Here is the site (https://www.mhlanguagelab.com/) that has Flash data - this add-on would not run the "Launch Web App" code.

doesn't work

This app doesnt work at all and seems to be a chrome app, this s because if you open the background.js it says "chrome.runtime.id" rather then the "browser.runtime.id" that firefox uses. This app also has a website "https://flplayer.net/welcome" which is the reason your being redirected to adfly then to a spam site for them to make money off of you.

Edit/Update: It doesn't work at all lol

On first install opened a new tab to some random website (https://themusic.io/).
Official website claims to be flplayer.net...

Seems suspicious, possibly Adware.

‘can't use it

..

doesn't work at all