You can't login to pocket on kobo with your Firefox account (at least at the present); you have to use a separate password. I had my Firefox account before I used pocket or had a kobo, and since I used that to login on my laptop, I had to do as in the below link to enable pocket to work with the kobo. This was long ago, so I'm not sure it still works, but perhaps give it a try.
See
https://help.getpocket.com/article/1...n-i-create-one