More detail please. Are you running McAfee? If not, you need to see what process is generating the popup in order to stop it. Try these tools from MS to see what's running when the computer starts and what process is the parent of the pop-ups.

Autoruns This shows you what will run when windows starts.

Process Explorer For this one, just click and hold on the black cross hair icon in the menu bar and then hover over the pop-up and it will then show you the parent process for that pop-up.

May the Schwartz be with you!

Also, not a Firefox issue, but user issue.  ;-))

I have not been able to find the cure .

If these are small popups on the bottom right corner of Firefox windows then these may just be web push notifications from sites you have allowed to send you notifications. Instead of useful information, some may send you junk, spam, fake virus/trojan alerts etc. You can revoke the sites from sending you notifications as shown in second link below.

https://support.mozilla.org/en-US/kb/push-notifications-firefox https://support.mozilla.org/en-US/kb/push-notifications-firefox#w_how-do-i-revoke-web-push-permissions-for-a-specific-site

Are you getting similar popups? https://support.mozilla.org/en-US/questions/1545503

I ended up finding a box to just turn off ALL notifications & apparently some other stuff . No way to possibly learn what site triggered it . Since then , when some legit sites , like Disqus on my own CoSy.com required permissions I gave it . But I still wonder if there are some sites or processes still pounding away to try to post spam notices .