<< Why it doesn't use stored credentials just like Sync does? >> If you sign into Pocket with your Firefox Sync account, this is the behavior. It should only

<< Why it doesn't use stored credentials just like Sync does? >> If you sign into Pocket with your Firefox Sync account, this is the behavior. It should only prompt you to re-login if you've cleared cookies. Even then, you can re-login via FF Sync without entering the password in again. If you're logging in with a service other than FF Sync (ex: Google), I'm not sure how you expect FF to know what your credentials are to log you in.