on the main page it seems that the content (gets shown within a widget sometimes) gets loaded insecure. The devs may change this because there is a secure image on same path.
So this is a problem of this instance.
On some other pages you'll come around insecure content, that may be an image within posts (which NodeBB shoudn't filter, so it's ok).
Also some profile-images on this instance have an insecure reference (for some users who registered in early ages of NodeBB, it won't happen with newer user-images, so it's an instance problem too).
Besides this problems I haven't encountered any mixed content yet (but I may have overseen them since I use chrome where the symbol is decent), so in your own board-instance you'd probably not encounter any mixed content (besides within user-posts).