The "Capture" button in EndNote Basic is not currently working in Firefox - no citation window pops up.
Please try the "Capture Reference" button instead, located here:
(You must login to your EndNote Basic Account before clicking on this link)
To install the Capture tool, just drag the Capture Reference button to your Bookmarks bar (also known as "Favorites Bar" or "Bookmarks Toolbar").
In some browsers, you may need to right-click and select "Add to Favorites" or "Bookmark This Link."
To use it, browse to a page you like and click the Capture Reference button in the Bookmarks bar. The Capture Reference window will open. Follow the instructions in the window.