I'm not sure it will help, but I also use Firefox and had similar strange popups on IMDB, Photo.net was losing some images on the galleries leaving blank spaces as well as having popups, and my Yahoo home page spontaneously switched to a totally new style that I hated over the classic. When I did a google search to figure out how to change back on Yahoo, it advised clearing all cookies, history and caches. I ran CC cleaner (filehippo.com) plus removed recent history and now everything is back to normal including all pop up issues. Good luck.
"One of the greatest necessities in America is to discover creative solitude." Carl Sandburg
I use NoScript when I'm browsing with Firefox. I don't know why anything would pop up on the forum, but NoScript would catch the culprit. Check your extensions and add-on list, and see if there is anything odd on it.
"It's the way to educate your eyes. Stare. Pry, listen, eavesdrop. Die knowing something. You are not here long." - Walker Evans
Next day, Firefox, no popups at all for me...
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