I'm sorry but this is a Mozilla Firefox support forum so unfortunately we cannot help you. You can try contacting your Email Provider about this issue.

The validity of email can be checked either by that it is of format *@*.* (* = any characters) or by an attempt to send something to it, so there are 2 solutions, 1) take another email address, 2) email the webmaster.

However, if the webpage accepted the email prior to the installation, then:

• Disable .net addons in Tools > Addons (check both Extensions and Plugins sections). Then, restart Firefox for these changes to take effect, and check for the issue.

• If the issue persists, uninstall the .net framework in Start > Settings > Control panel > Add/remove programs, and try to input your email again..