I recently had an issue with a new tab opening upon launching Firefox that was offering an explanation of logging into clipper. I solved it this morning and thought someone might benefit from my experience.
I have had cookies blocked in Firefox for years with little negative impact, however this was the culprit in this case. Looks like Evernote Clipper keeps login credentials in a cookie. To keep the security settings almost entirely in place by rejecting all other cookies, you need to provide an exception for the Evernote cookie and change one other setting in Firefox to retain this configuration setting.
First: Options>Privacy>History section.
Enable: Use Custom settings for history, Accept cookies from sites, Accept third-party cookies: Never, Keep until: they expire.
Click the Exceptions button and enter: .evernote.com and click the "Allow" button. Save Changes.
Hopefully I've correctly documented what I did and what worked for me and hopefully it helps someone.