The days of native popups in browsers are long gone, but sites still frequently use in-page alerts like the one above. They are completely useless most of the time, and usually violate the Better Ads Standards (potentially causing the site to lose revenue from Chrome users). While Firefox isn't outright blocking ads on some sites like Chrome is, the browser's developers are working on a blocker for these popups.
The feature is still in the planning stages, but Mozilla is asking users for any examples of sites with annoying pop-ups. Unlike uBlock Origin and other ad-blockers, which use a predefined blacklist that must constantly be updated, Mozilla wants to make Firefox automatically detect and dismiss the popups.
If you know of sites that use in-page popups (whether it be newsletter signups, surveys, or something else), you can fill out the survey here. There are also Firefox and Chrome extensions that make the process easier. I'll be interested to see how Mozilla pulls this off, it will no doubt be difficult to detect the difference between helpful and not-helpful popups.