Can you please give the exact error message? Also, update to Firefox 57

Did this work before in older Firefox versions? Unless the firmware of this router is that old that is doesn't support current releases then it should work. Firefox 51+ will mark HTTP connections with a padlock with a strike through and shows "Connection is Not Secure" if you click the Control Center 'i' button at the left end of the location/address bar. <p>Note that this message is meant as a warning that you could be vulnerable and that it shouldn't prevent (block) you from accessing the website.</p> <p>You can press the ESC key to close a doorhanger with a warning message in the name and password field.</p> <p>See also:</p> <ul> <li>https://support.mozilla.org/en-US/kb/how-do-i-tell-if-my-connection-is-secure</li> <li>https://support.mozilla.org/en-US/kb/insecure-password-warning-firefox</li> <li>https://support.mozilla.org/en-US/kb/what-does-your-connection-is-not-secure-mean</li> </ul>