The first image shows the index page. All the images are HTTPS so all is well, and the red arrow points to the secured lock icon which is the indicator on firefox that the page is secure.
Second image shows a Signature image using the Signature extension, the image a HTTP Imgur link.
Its my understanding that the HTTPS extension should either not display the image and give a warning why or it should proxy the image through https.
I tested also by creating a new post and adding the same image as above. Upon refreshing the post the image WAS proxied through https. So I believe the issue with the Signature extension.
Anyway your extension could blanket all of the forum including other extensions?
Thank you for your time and your awesome extension ❤