It is not working on some pdf's viewed on firefox. The button don't display anything. For example in https://www.researchgate.net/profile/Peter_Driessen/publication/242097053_Path_Difference_Learning_for_Guitar_Fingering_Problem/links/5460b9d70cf295b5616375b4/Path-Difference-Learning-for-Guitar-Fingering-Problem.pdf

UPDATE
Now if I use Mendeley Web Importer on a PDF the following message appear: "For security, Firefox blocks extensions running on PDF files".

It is a pity that the extension does not work in a browser as good and safe as Firefox. It is time to go back to Chrome again.