Since Firefox 33 a “Secure Connection Failed” error can no longer be bypassed using the “I Understand the Risks” button. That buttion is gone! It turns out that Firefox now requires self signed certificates to be 1024 bits or longer where older versions only required 512 bits. Therefore you need to make a new certificate that is long enough.
You can fix this right from Webmin if you use another browser (such as Chrome or Chromium) that lets you bypass this type of error . Otherwise you can log in to your server and temporarily disable SSL in webmin by setting ssl=0 in /etc/webmin/miniserv.conf and restart webmin with “/etc/init.d/webmin restart”.
Then, either way, you can login to the Webmin web UI and select: Webmin -> Webmin Configuration -> SSL Encryption -> Self Signed Certificate. Fill in the form (or leave the defaults) and then click the Create Now button. This will update your self-signed certificate for Webmin and you will now be able to access the page from Firefox 33 (With the usual browser warning about an untrusted connection).
If you temporarily disabled ssl you will need to re-enable it with ssl=1 in /etc/webmin/miniserv.conf and restart webmin with “/etc/init.d/webmin restart”.