Save YT Songs To Spotify by mattx2hias

This extension allows you to search and save songs you find on Youtube to your Spotify, Liked Songs playlist. The popup will display up to 3 search results.

How to use
After installing the extension, click on the icon on your menu bar; this will bring you to the authorize prompt. Upon accepting, you'll be redirected to the Github repository. From now on, with any Youtube video you have up, the extension will search Spotify with the title when you click the icon on the Firefox toolbar.