Using an outdated version of mail.com MailCheck could cause errors in the extension.
Generally, MailCheck automatically updates itself. In some cases however, you may have to update MailCheck manually.
MailCheck will be updated to the latest version.
Click the menu symbol and then click Add-ons.
This will open the Firefox Add-on Manager.
Right-click mail.com MailCheck.
A menu will appear.
Click Find Updates button.
If the message An update is available appears, click Update Now.