If you press F12 in either Firefox or Chrome to enable the Web Console, it will tell you which parts of the page are insecure.
In some of those threads it is because of http:// links in signatures. Other threads are pulling content from various insecure sites, but I'm not sure why.
Edit:
According to the source code, the following users are displaying an invisible avatar from an insecure link. All these users were registered back in 2011 and have lain dormant until 2018.
leholmes12 saymajaan24conklinlianeArtHawk678RalphPittsclark581It is explained in mprep's old post from above:
It's not fully secure since it's linking to an avatar located on a website without SSL (as in an image outside the forum). Seems like one of the few remaining users (specifically
this guy) who got an avatar before the avatar vulnerability was discovered and hasn't changed it since. I'll ping theymos about this.