I found that this add-on interfered with several websites that I used regularly. One was the Washington Post site. Also NYTimes and IMDB. When I turned off PopUpOFF, things on these sites went back to normal.

I can't say that this app is the only contributor to my problems, as I have other add-on blockers installed as well. Also the problems I had with websites evolved over time, so there may have been contributing causes from website changes. But when I stopped this add-on, problems disappeared.

completely breaks twitch.tv for the most part and hasn't been fixed in the past year

For some reason, this add on seems to "break" ebay's purchased items "tracker". Turn it on and it takes forever to open the pane where you can see your shipping status and then it takes an equally long time to close it after you're done. It's not my hardware as I'm running a very new and very high spec'd MSI gaming laptop. It's lightning fast on every web site and program I visit/use. Removed the app and ebay site was back to normal behavior. Tried several other pop up blockers and none of them break ebay. Currently running "pop up blocker (strict)" and no negative effects on any web site. It's possible firefox or ebay changed something recently, as I used this app for about a week without this problem. Kinda scared me that I may have picked up a keylogger or virus somewhere, but MaCafee and Malwarebytes came back with a clean report after a couple scans. So, unfortunately, I can not recommend this add on while it has this limitation, at least not if you use ebay much.