Did you try resetting Firefox? If not ....
Open Firefox and go to Help > Troubleshooting Information. In the next window click on Reset Firefox. Click Finish when it's complete.
If this is the same pop-up referenced in your post titled "Pop up ads in Firefox", I would kindly ask (in the future) not create a second thread with the same issue. It tends to create confusion and also duplicates work. Thanks!
Best of luck..
I keep getting pop up ads from "Deals" which states it's "brought by Plus-HD-1.2" which I Googled and it appears it's malware but I can't find it on my computer via control panel and malwarebytes and adwcleaner neither one found it either. I have firefox with ad blocker add on with the easylist. At a loss what to do...please help...thank you.