The person you responded to specifically explained that. It's made to look non-native on purpose so that users don't think a pop-up, phishing, asking for their password, is a native system prompt. Safari did the same thing. It's not that it's neglected, quite the opposite, actually.
Personally, I don't think the Firefox version looks any worse than the Safari version, they're just different. It comes down to preference. Just like you saying it's the ugliest tab bar which I very much disagree with.
When I say polished I don't mean a polished visual style in an aesthetic sense. What I mean is that a designer has put some work on a UI element and after some iterations there has been a conscious decision over why an element looks the way it looks.
For example, one of the major considerations when doing UI design is creating elements that have a cohesive style (regardless of which aesthetic style that is).
None of that happened on that god forgotten alert. It's probably the first thing that someone came up with in a rush, in a style that no other UI element of FF shares, and it has stayed like this probably because nobody cares.
