Hi emm54chick, what is the message you get when a site is blocked? For example, does it mention that Firefox can't make a secure connection? If you could, please copy/paste the text of the message here, or you could share a screenshot.

Also, I don't think Firefox would normally suggest downloading an app on Windows, since Firefox itself should be able to connect to websites without any add-ons. It sounds a little suspicious, like maybe an attempt to infect your computer with malware. Do you recall more about that?