The post is quite an old one and needs an update for HTTPS. Pls use CSS which looks like below to make it work seamlessly across both HTTP/HTTPS.
@font-face {
font-family: 'MuseoSlab-500';
src: url('//mydomain.com/wp-content/webfonts/26B0C6_0_0.eot');
src: url('//mydomain.com/wp-content/webfonts/26B0C6_0_0.eot?#iefix') format('embedded-opentype'),url('//mydomain.com/wp-content/webfonts/26B0C6_0_0.woff') format('woff'),url('//mydomain.com/wp-content/webfonts/26B0C6_0_0.ttf') format('truetype');
}
Basically, all we are doing here is to omit the http/https prefix for the URL. This way the right prefix will be used depending on type of request.