I have this same behaviour on Mozilla Firefox, all versions up to and including 44.0.2 on Windows. So browsed into this forum. The Web Clipper just stops working in Firefox browser windows. But sometimes I can open the target URL a new window, and then the Clipper works fine from a new, freshly-opened window. But sure would be nice if Evernote could fix it so the Web Clipper add-on works 100% of the time!