Hi, is it this one: https://addons.mozilla.org/firefox/addon/mendeley-web-importer/ Does its icon appear on the list that drops from the Extensions (puzzle piece) button? The following article has more info on that: [[Manage your extensions using the extensions button in the toolbar]] If it is not listed there, click the text at the bottom to proceed to the Add-ons page. Is the extension listed there in the Enabled section? If it's in the Disabled section, click its slider switch to enable it. If it isn't available even after enabling, is it possible you are browsing in a private window: [[Extensions in Private Browsing]]. Let us know how it goes.