This happens to me also. Evernote first launch opens on Firefox everytime I close and run firefox. I have the Evernote web clipper extension installed and I have to login everytime. I uninstalled Evernote web clipper extension and the Evernote first launch page does note open. Obviously, the web clipper extension has something to do with this odd behavior. The web clipper extension requiring to login everytime firefox is run also opens the Evernote first launch site. I hope Evernote can look into the extension's compatibility with the new firefox version 50.0.