You say that your pop-up blocker isn't working properly and then you blame Firefox ..... Truth is : you've got malware. Go to the Firefox menu (≡) => Add-ons => Extensions

( Mac  : Command + Shift + A)

and look for any other unfamiliar or suspicious looking extensions.

When you find any : remove them.

Do the same here : for Windows => Control Panel for Mac and Linux => System Preferences

However : most malware will be 'hiding' somewhere , so let's run malware scans (even though you already have run some) :

Further information can be found in this article : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. They all work differently - what one program doesn't pick up, the other might.

Also see : https://discussions.apple.com/thread/8168100

Especially for Mac : https://www.malwarebytes.com/mac/

And : https://malwaretips.com/blogs/remove-mac-os-x-virus/

I can't find any malware on my Mac, nor can I find any of the scans you allude to. Xfinity keeps getting in my face with their pop-ups, worse and worse, but ONLY when I'm using Firefox.

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jonikayrose said

I can't find any malware on my Mac, nor can I find any of the scans you allude to. Xfinity keeps getting in my face with their pop-ups, worse and worse, but ONLY when I'm using Firefox.

Those scans you'll find in the first link I gave you :

https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

And I added two links to scans especially for Mac ....

Did you read what it says in this link :

https://discussions.apple.com/thread/8168100  ?

It will show you that you're not the only Mac user who gets these Xfinity pop-ups, and maybe what did the trick for that user, might work for you as well.