I have a feeling that’ll be embedded images in some posts or something like that - but I’m not sure where they are.
I don’t see that on this thread (FF 53 on OS X).
Firefox 53.0 on Ubuntu 14.04 is fine here.
FF53 (64 bit) also OK on Ubuntu 16.04 here.
Now okay here ¯\(ツ)/¯
I had intermittent problems with Silicon Canal when I was struggling to get registered there.
I think it’s something that pops up after spending a little while on the site, maybe after viewing a post with an embedded image from a non-https source.
It’s what Daniel said, e.g.
Though it seems quite hard to find now, it was worse in the past. Anyway it’s not due to the certificate change.