Go to the Mozilla Add-ons Web Page {web Link} (There’s a lot of good stuff here) and search for a good ad / pop-up blocker.

Sorry about the screenshot uploading, I don't know why that is so inconsistent. You can attach the image to a reply: https://support.mozilla.org/questions/1160228#question-reply.

First, and most importantly, the built-in popup blocker only blocks completely distinct popup windows, it doesn't block in-page content such as overlays, boxes that move around as you scroll, etc.

If in-page content is a regular annoyance and generally contains unwanted advertising, you might need to supplement the popup blocker with an ad blocker. You could consider these popular options (but only run one at a time for performance reasons):

• Adblock Plus: https://addons.mozilla.org/firefox/addon/adblock-plus/
• uBlock Origin: https://addons.mozilla.org/firefox/addon/ublock-origin/

Second, there are popups that are script alert messages or script prompt dialogs. These generally have only an OK button or OK and Cancel buttons. The built-in popup blocker doesn't block these either, and neither would an ad blocker, but these are rare.

Thanks to both FredMcD and jscher2000. I did change from uBlock Origin to AdBlock Plus, and the pop-up has not reappeared so far.

I was successful in uploading the image this time. Doesn't it look like a phishing scam? There is nothing to indicate who is initiating a chat session.

If you don't know who it's from, don't answer. It could be anything from porn, ads, or other things you don't want.