Copied this highly useful info straight from this website. Very strange that I didn’t have this problem until I hit Firefox version 39 but it is possible that I just hadn’t administered that particular server for a long time in Webmin.

Recent browser versions (e.g. Firefox 33) refuse to work with older Webmin installs.

They give a sec_error_invalid_key error, offer a ‘Try again’ button, but do not offer an option to add an exception.

Firefox 33 no longer supports certificates with private keys smaller than 1024 bits.

You can replace your webmin certificate with a new one by running this command: Continue reading “Firefox – Version 33 Update Causes Certificate Error in Webmin”