For example, Christianity Stack Exchange is running an ad for the Hebrew Language proposal. This causes Firefox to give security warnings.
Connection is Not Secure
Parts of this page are not secure (such as images).
You just need to update the community ad post to use the HTTPS image. i.e...
Not sure why the automated script didn't catch those, but the HTTPS ad images work so it's a simple fix. The links generated by Area 51 have been updated so this shouldn't be an issue with any future posts, I guess there may be a few existing ads that need updating though.