Avast web security keeps giving pop-ups on

I got a pop-up about an Adult website, and restarted Firefox after that. History and cookies and so on have been deleted, but I haven’t reinstalled Firefox (yet).

Now I keep getting that same pop-up whenever I use Firefox. I have not been to the same website again; the pop-up last occured when I was still on google.com. Doesn’t seem to happen in Chrome, or at least hasn’t happened yet. Avast boot scan (set to search everything for anything and with highest sensitivity) finds nothing malicious, corrupt or other, nor does Malwarebytes.

Pop-up says the following every time:
Name of threat: Adult [TrJ]
Type of threat: Adult
URL: https://sexyforum.to/
Process: C:\Program Files\Mozilla Firefox\firefox.exe
Found by: Web protection
Status: Connection aborted
57103347ac08c/211653.1412+0200

Please advice on how I get rid of this.

nor does Malwarebytes.
Have you tried Malwarebytes Adwcleaner https://www.malwarebytes.com/adwcleaner

Do you have Adguard or uBlock that will block out all adult context from your browser, plus you need to completely clean out your cookies from your browser as well.

@ Yewoy
First break the link to a suspect site in your post to avoid accidental exposure (or promotion by web spiders) change it to hxxps://sexyforum.to/

Secondly having cleared the browser cache and cookies, if it still persists, run firefox with add-ons disabled.
Does it still happen ?
If not then there is one of possibly more add-ons connecting here. So try disabling only those add-ons recently added or update does that work ?