This is determined entirely by the website, not Firefox. Firefox simply loads what the website tells it to. For this reason, it's a little difficult to get the desired performance across all websites. Some content blocking (ad blocking) extensions available for Firefox may block it for you but, since it's not really an ad, a lot will leave it visible. My only concern is if it's asking you to log in on literally every website you visit. In that case, there is a possibility that it's a fake phishing popup possibly added to the page by malware or a browser add-on. If that's the case, perhaps you should [[Troubleshoot Firefox issues caused by malware|check your computer for malware]]. Hope this helps.