You can try to reinstall the autofill plugin for Firefox:
Please navigate to the main Sticky Password user interface window - menu - Settings - Supported browsers and check if Firefox is listed as "Installed". If it´s not installed, install it by pressing the "Install" button. If it's installed, uninstall it by pressing the "Uninstall" button and then reinstall it by pressing the "Install" button".
Also verify if you are using the latest Sticky Password 7 update. Currently it´s the version 7.0.7.
You can delete an account directly in the main user interface window - Web Accounts.