Since the update of the webclipper this does not work anymore:
when I had clipped a webpage, then the clipped note opens in a new local (!) Evernote window, so that I can file it in my desired Evernote notebook.
At the moment it is filed automatically in the standard notebook. So I need an extra step: going to my local Evernote, going into the standard notebook and filing the clipped note into the right notebook.
This happens ALTHOUGH I have set the Evernote (for Windows 7) settings to "open clipped note in a new window"!!

(Firefox 50.0, newest Evernote for Windows 7)

I installed this add-on and like so many others I got VERY frustrated with the "unknown error" in the sign-in box.
So I did some googling, and found the suggestion regarding AdBlock below (adding the @@||http://evernote.com^ exception); however, this did not help.
So I went one step further, and completely disabled AdBlock.

The sign-in worked flawlessly.

After that, I figured I'd re-enable AdBlock, only to find that suddenly WebClipper thought I had no Internet access. So I pulled up the filter preferences, and searched the whole set of list subscriptions for 'evernote.'
I believe there were four different entries, not including the exception rule I'd made.
Disabling those 4 list entries solved all the issues I was having.

I hope this helps other users that are frustrated with this add-on and that the dev's take a moment to let these frustrated users know that the add-on is 100% usable, just not automatic when paired with several (extremely common) adblocking add-ons and lists that are out there.

Hopefully there's a way to adjust the add-on's permissions or path such that it doesn't get stuck in this dead-end. At least a "no connection available. Are you connected to the internet? are you using an adblocker?" dialog that directs users to an Evernote how-to for disabling adblocking for Evernote functions.
Thanks for making this app!

uBlock origin (and other ad blockers using adblock rules subscriptions) block work Evernote Web Clipper.

Reason filter "BitBlock List" it contains a rule:
||evernote.com^$third-party

If you disable it, the problem goes away. You can add a filter permitting.
@@||http://evernote.com^

I hadn't updated for a while, but noticed I needed to login.
Unfortunately I get an "unknown" error every time, making web clipper useless.
I tried updating everything, Firefox, Evernote, and web clipper, but same result.

Impossible to log in, when you click the add-on icon, a dialog box appears with two blank text fields, a blue button and a green button - no explaining text whatsoever, and neither button does anything when clicked.
I'm considering cancelling my premium membership, Evernote doesn't seem to be listening to its customers any more.

The new version does not clip local web pages (HTML-Files) anymore! This is not considered as a bug but made by design: Whenever I tried it I got a message that local files can't be clipped.

I always thought that new versions should be improve performance and functionality, but not so with this version…

The new version seems to introduce some new features, but I no longer stay logged in as I did with previous versions...which means logging in (with 2-step authentication) every single time I open a browser window. Paid subscription here, and admittedly a little frustrated.

it's just impossible to log in...

same err with @xRay's, except that I'm using Arch Linux.
After I upgrade Firefox to 50.0, I get this err every time I try to login.

If I could give it .5 star rating I would everytime I open firfox evernote opens a tab, I can not get it to stop doing this..it is frustrating!!

Have to enter username/password everytime I start Firefox. As a bonus, an Evernote page opens. Unusable.

New release of the web clipper since yesterday.
As a result: doesn't work any more.
As I'm premium, I have paid-equal I lost my money (and maybe 2 hours this morning).

It is doing everything I need, hassle free and fast.

After updating, it doesn't work anymore.

Removing the option to save a clip on the local PC is the dumbest, and most ridiculous idea any fool with a double digit IQ could have come up with.
Not everybody has access to unlimited bandwidth to sync up and down all the time.
You have managed to totally destroy a very good application.

It's broken with the last actualization. And it looks like this is happening to everybody.

J'avais l'habitude de l'utiliser jusqu'à hier...
Après une réinstallation système, il ne veut plus s'ajouter à firefox.
Il bloque indéfiniment à la vérification...

Très pratique quand il fonctionne, mais là ça m'énerve.

Evernote Web Clipper was updated today.
Suddenly it needs my password and yet does not
recognize it. This upgrade has broken the add-on entirely. Useless.
Great job.

何かブロックされているらしく、クリップできません

Совсем испортились! Сначала отказались от Clearly, теперь поломали и дополнение.
Пользуюсь этим дополнением уже 2 года. Все было нормально. Последние два дня при каждом открытии Firefox выдает страницу руководства и забывает логин и пароль. (последние две недели ничего в настройках не менялось)

Evernote produtcs became worse and worse. Recently they drops Clearly. Now the Webclipper became more buggy.
I've been using this addon 2 years without any problems. But last couple of days it forgets login/password and shows me first page-about-webclipper on EVERY Firefox start.
(I've not changed any settings last two weeks).