I confirm the malfunction warning issued by the previous user: once installed, the add-on didn't work on my machine either... the small icon didn't popup near the selected text. Not understanding why, I went to the add-on's permission settings and discovered that the "Access your data for all your websites" permission (in French on my machine) had remained deactivated. I activated it manually... then I refreshed a page that was already open in any other Firefox tab, and tried a translation again: this time it worked, the small icon poped-up and the translation appeared when clicking the icon. And since then, everything works perfectly. I don't know whether it's Mozilla that refused to allow this option to be activated by the add-on installer or whether it's an oversight in the add-on installer itself. Anyway, everything now works perfectly.