A very useful & need to have add-on when using your Firefox browser. I actually use two different ones to cut down on unwanted spam & ads which work great together & can both be found on my home page as I promote the ones I like.
I had no pop-ups sneak by this extension in the wild. When testing it against a controlled environment from a website, it failed only on the drop-down popup. The extension is strange in that is needs Firefox's default pop-up blocker activated but does seem to fortify or add additional rules on top of Firefox's pop-up blocker. Deactivating the browser pop-up setting basically disabled the extension as well. There is no seamless way to add individually to the white or blacklist, only whole domains. Clicking on the yellow notification strip to add to whitelist sends it straight to Firefox whitelist settings. You have to copy & paste into the whitelist or blacklist for individual pop-ups. If you simply want to block pop-ups, it does a good job but needs little polish for features.