You can delete your cookies and website data for a single domain in Firefox for Android, without losing your logins for other domains.

To delete your cookies and website data for a single domain:

1. Go to a website that requires cookies.
2. Accept the cookies.
3. Tap the Lock icon in the search bar.
4. Tap Clear cookies and site data.
5. Tap Delete.
6. Reload the website and you should see the prompt requesting you to accept cookies shown again.