Read if you can't Login with Firefox!! Rated 5 out of 5 stars

In Options, go to the Privacy tab, Next to "Accept Cookies" click Exceptions, Enter: evernote.com and select Allow. It should work. For some reason if you don't enable 3rd party cookies it doesn't work. I don't allow them, so this exception helps. Works on 18.0.1

This review is for a previous version of the add-on (5.4.1).