block popup windows

In Firefox, go to Settings → Privacy & Security → Permissions and make sure Block pop-up windows is checked.

You can also click Exceptions… beside it to see whether a particular website has been allowed to open pop-ups. If you’re still getting pop-ups after that, they may not actually be browser pop-up windows. Some sites use notifications or new tabs instead. In that case, tell us what kind of pop-up you’re seeing and we can narrow it down.