"Password Required" pop-up

I am basic user with no knowledge about code. There I got that out of the way.

Lately when I load my website in Firefox, I get a pop-up “Password Required.” You can then click “ok” or “cancel” and everything is fine. But this website is for my business and I don’t want potential clients navigating away because of this error.

I have tested on Firefox, Internet Explorer, and Safari, and this pop-up only occurs on Firefox.

Any thoughts?

Thanks,
Jeff