Remove any stored logins that may be incorrect with the Password Manager: Password manager - Remember, delete, change and import saved passwords in Firefox

Clear the Firefox cache for the site that you are having this issue with to removed cached pages for unsuccessful logins: about:permissions and click "Forget this Site"

This issue can be caused by corrupted cookies or cookies that are blocked.

• check the permissions on the about:permissions page and in "Tools > Page Info > Permissions"
• https://support.mozilla.org/kb/fix-login-issues-on-websites-require-passwords

Clear the cache and remove cookies only from websites that cause problems.

"Clear the Cache":

• Firefox > Preferences > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox > Preferences > Privacy > "Use custom settings for history" > Cookies: "Show Cookies"