Hello,
I also installed the brilliant “Let’s Encrypt” SSL certificate as my web host also includes it in their CPanel.
But I had a problem with the yellow triangle and exclamation mark warning from Firefox, saying that some parts are not secure.
Another problem I had was that on the Opera browser, if I did a search on my site, when I reached the Search Results page, it would not be secured and the green lock disappeared.
I found a quick fix for now by using a plugin:
https://wordpress.org/plugins/really-simple-ssl/
Because the warning was still showing in the WP admin area, I also added this other plugin which resolved the issue:
https://wordpress.org/plugins/wordpress-https/
I came to this forum to see if SSL was mentioned and I found your post.
I visited your site on Firefox and I see that you also have the same problem.
If you click on the yellow warning, you can see that the problem is caused by some elements like images’ urls being http instead of https
I think the plugins are a quick fix, but it’s probably a better idea to change all files on the system to https and I’m looking for a way to do this.
I have another problem now. If I activate the GD Booster addon, then the warning shows again…
I have attached a screenshot of the warning icon on Firefox.