If I try to access the forum using https in Firefox, I'll get an insecure connection warning, but that is because the site isn't configured to use https outside of account settings.
What's likely triggering it is the fact that this is an http website that has numerous links and pictures to outside sites.
The internet as a whole is insecure. It's based on protocols written in the 80's, and they were designed to work and be robust. Security wasn't a big issue then, because the internet was still a pet project and it just needed to work. Then the www boom hit in the mid-90's, and the protocols were never changed to be more secure. To this day, security still seems to take a back seat to speed and convenience. That lack of security leads to something along the lines of what the show Person of Interest was about:
https://youtu.be/EzG4a9jTxGw?t=3m44s