As the subject says. That's the new message you see under "about:addons" when you have Firefox 40.0 installed.
It's no surprise given the release notes from Mozilla https://www.mozilla.org/en-US/firefox/40.0/releasenotes/ "Add-on extensions that are not signed by Mozilla will display a warning"
Screenshot;
This could cause a lot of unnecessary forum posts. Before you say it, Yes I have the toolbar patch for Firefox 40.0