I cannot successfully log in to Evernote Web Clipper v.6.13.1 in Firefox 57.0.3. This is because of the design of the log-in pop-up window. It prompts me for my username first, then requires me to press the 'Continue' button. It should then make the password prompt box visible, but it does not. On every third time I click the 'Continue' button, I see an unformatted version of the complete log-in page (with password box) flash onto my screen for a second, then back to the username prompt. So I can literally not log in. This fails to impress/surprise me in equal amounts.
Thanks to the advice offered above, I installed Web Clipper v.6.9.3. This does allow me to log in. It has the older user interface but is still compatible with Firefox Quantum. (Whitelisting the evernote.com domain in my adblocker and cookie deleter add-ons did not solve this issue). By reverting to an older version successfully, this indicates that the feature used to work, can work with FF57+, but some recent coding changes have messed it up.
When I tried to log in to this discussion forum to post this comment, using the same Firefox 57.0.3 and downgraded Web Clipper combination, I was presented with the same problematic log-in page. So I literally had to switch browsers to log in to post this comment about not being able to log in. The problem is with your log-in form layout! Why get all fancy with the 'Continue' button? Why not just show the password box and let me get on with it?