It's a hint for outdated browsers, it's not an ad and there's nothing malicious behind it (unless there's an issue in your local network or machine perhaps?).
I think trying to emulate a built in browser notification is the wrong way to go. I wouldn't have got as concerned if the popup looked more like a web page overlay, with wording along the lines of:
"Hi we're from Browser-update.org (clickable), and this site has asked us to check your browser version. Your browser version is out of date, and should be upgraded. Please go to your [settings] menu to update, or visit [us] for more information."
Of course, feel free to totally ignore my suggestions - it's a free world. ;)
http://browser-update.org/ https://www.npmjs.com/package/browser-update
It's a fine line though to not get into the way with things like this, and we could certainly improve it's appearance a bit - sorry for that.