Thank you for your feedback.

Unfortunately, we cannot reproduce the issue mentioned above.
Is it possible to login directly on the gmx.net portal? Do you get the same message there?
Do you use other extensions, i.e. NoScript, that block website content? In other cases this caused similar behavior.

Please send us an e-mail to mozdev@1und1.de. Also we would be grateful to receive more details as follows:

1. MailCheck version
2. Firefox version
3. Error message (if exisiting)

Providing screenshots or videos could be helpful to solve the problem quickly.

Best regards,

Your MailCheck team