This add-on downloads .mp3 files from the linguee dictioary once you visit the page in your browser. The downloaded audio can be found in your browser's download folder, within a language-specific folder.
Example: if you are visiting deutsch-english translation pages on linguee, the audio will be saved under deutsch/some-word.mp3
Git clone this repo to some local folder. Then fire up Firefox and in the address bar type:
about:debugging
In the tab that opens, choose 'Load temporary Add-on' and navigate to the manifest.json file that is in the root of the repo. Select it and the add-on will be installed for your current firefox session.