This happened earlier today, which is when the screenshot below is from, and lasted only a minute or so then. With the issue quickly resolving itself I didn't go through with the bug report I was working on at that time. But, with the pop-up coming again just a few minutes ago, I decided I should ask to figure out if it is a client or server-based issue.

The pop-up would appear every time I went to another page of BK. It was easily exited out of and I could navigate on the page as usual afterwards but, after clicking to another page, it would pop up again until it just stopped doing so as quickly as it started.

It's a client side issue.

saffplg.js comes from McAffee Site Advisor. Here's a discussion on Mozilla Support Forum:

https://support.mozilla.org/en-US/questions/748889