Is that part of the web page? You may be able to tell by right-clicking the blue background and checking whether you get a typical Firefox right-click context menu. Let's assume it is. If it works like Google's similar messages, the site sets a cookie when you close that "ad" and as long as that cookie persists, you wo't see it again. Judging from the tracking protection shield icon in the address bar, you may be using automatic private browsing, so then that cookie would be gone on your next visit to the site. So you may need to use a different mechanism to suppress it. One possibility is to use a feature of an ad blocking extension like uBlock Origin to remove it. See: https://www.ghacks.net/2017/02/21/ublock-origin-how-to-remove-any-element-from-a-page-permanently/ Another possibility is to use a custom style rule, but someone would need to figure out the details of that dialog in order to write it for you. Or perhaps someone has posted one somewhere on the web...