For Plugin issues you need to check to see if it is update to work with the latest version of firefox browser from the developer. If not then you have to contact the developer to see if they are going to update so it works with latest browser update. firefox isn't responsible for addon/extension developer for not updating their addon/extensions.

See the most recent reviews of that Add-on. https://addons.mozilla.org/en-US/firefox/addon/duokeyboard/reviews/

Two users have reported problems in the last day with that Add-on.

Lorenzo462 said

Hi, For many months I have been using the plug-in for the Duolingo russian Keyboard ... Why? Is Firefox making some improvements for this plug-in.

This is a Extension and not a Plugin. It is made by a third-party author and not made or maintained by Mozilla who makes the Firefox web browsers. The vast majority of Extensions listed at addons.mozilla.org are made by a third-party author or company.

WestEnd said

For Plugin issues you need to check to see if it is update to work with the latest version of firefox browser from the developer. If not then you have to contact the developer to see if they are going to update so it works with latest browser update. firefox isn't responsible for addon/extension developer for not updating their addon/extensions.

Hmm you are using the words extension, plugin and addon to refer to one thing.

The mentioned "Duolingo" is a Extension. A Plugin refers to say the Flash Player Plugin from Adobe for example which the only NPAPI Plugin allowed to run in the Firefox web browser now days. The word Addon is a general term (started during the Firefox 2.0/3.0 days) that groups together the separate Extensions, Themes (complete, image, color), Plugins, search engines, dictionaries, language packs.