Open Translate Addon by Terraprint

This extension is unofficial. It is compatible with any LibreTranslate instance but is not officially endorsed by the developers/companies behind LibreTranslate.

Allows users to incorporate the LibreTranslate API as a language translation aid in the Firefox browser. It has 3 major features: inline translation (cursor highlight and hold Alt key), traditional textbox translation via the popup, and the ability to choose which public instance of the LibreTranslate API to use or to self-host it

Source Code: https://github.com/K-Francis-H/libretranslate-unofficial-ff-extension