From what i know these screens are default on Chrome and Firefox and don't come in with an addon. Here is the screenshot. The warning appears especially if i'm coming from Google searches to Vanillaforums.org, because i usually search for forum entries with Google.
I have run into this problem before. Some search results on Google link to the https protocol of this site and changing the link to the http protocol would stop the error from showing up.
Great investigation. I just wanted to mark that, as probably some users who come here via a Google search will fall into this trap believing that Vanillaforums isn't such secure or something. Yo never know.