The most common reason for a login failure would be that a cookie was not set correctly. This could be because Firefox is not accepting cookies for the site, or the site's cookie is corrupted in some manner. The following article describes how to check your cookie settings to make sure they allow sites to set cookies. The Privacy section of the Options page also has an Exceptions button to view site-specific cookie rules to make sure the mail site isn't blocked: [[Websites say cookies are blocked - Unblock them]] To clear possibly corrupted cookies, try the Permissions Manager. To open that, type or paste '''about:permissions''' in the address bar and press Enter. Firefox should start listing a huge number of sites in the left column. Use the search box above the list to enter a few characters to filter it down to your mail site. Click the site in the list, and then, on the right side, clear the cookies for that site. If there seem to be multiple related sites, check each of them for cookies. Then try visiting the site and logging in again. Any improvement?