Its likely a browser extension or plugin, or a DNS hijacking
Originally Posted by danielaruiz-h
First, try running firefox in private browsing mode, do the popups appear then?
Then, if they still appear, start disabling all your browser extensions. If it dissapears then, then it is a browser extension causing the issue.
Try closing firefox and running
and open firefox again
mv .mozilla mozilla-bak
If that works without popups (note that your bookmarks and history, and rest of the stuff arent lost even if they dont show up in the browser), that means that it is a browser issue.
If its a browser issue, run the below, export the things that you need from Firefox
Then, run the below to rename the infected firefox profile, and import the bookmarks, history, .etc that you exported from the old firefox.
mv .mozilla mozilla-new
mv mozilla-bak .mozilla
The infected firefox profile is now named mozilla-old, and firefox should automatically generate a new profile for you
mv .mozilla mozilla-old