Help with: unsolicited pop-up while using Firefox

Hello all,

I have a strange issue where I am getting a random pop-up while using Firefox on my desktop. The pop up appears randomly on different web-sites at a frequency of once or twice a day. It appears in Chinese from a website called storm.mg. The only add-ons I have are a Facebook blocker, and a Zotero plugin. Other than that I can’t imagine where this pop-up is coming from.

Does anyone have any ideas as to where this pop-up could be coming from? Or tactics to get it removed?

Thanks!