You're encountering this problem because Firefox and IE do very poor error handling for AJAX queries and unrecognizable characters.

Your inbox likely has at least one message that contains unreadable characters in the subject line. This could be actually unreadable characters (gibberish essentially) or they could be another language that your web browser cannot understand. Google Chrome handles this by simply displaying the unreadable characters, while Firefox and Internet Explorer handle it by completely failing to display anything and providing the error you saw.

To fix this problem download, install and use Google Chrome, rather than Firefox or IE, to login to your webmail. Move the message to the Spam or Junk folder first, then erase them completely.

You will now be able to login using Firefox again, though you may wish to continue using Google Chrome for its fast browsing and apparently better error handling.