Pocket stores your password securely using Firefox's password manager. If you have configured a master password in Firefox, You will be prompted to enter it each time you open Firefox, since Pocket requests access to your login details so it can sync at launch.
You can do one of two things to avoid the prompts:
- Disable your master password
- Disable Pocket's 'Store Password Securely' option. To do this, open Pocket, click Settings, and select Advanced. Then, disable 'Store password securely'. This will store your password in a different place that does not require Firefox to ask for your master password.