所有流行的浏览器的用户代理字符串，甚至Internet Explorer的，都以Mozilla/开头。为什么会这样呢?
当前回答
这是一个漫长而悲伤的故事。
总而言之:
Mozilla browser gets released, with User-Agent Mozilla/1.0 (Win3.1). It is publicly renamed to Netscape, but in its User-Agent it keeps its original name . Internet Explorer is released. It spoofs Netscape by starting its User-Agent with Mozilla/ because web servers were routinely browser sniffing and serving pages with frames - a feature supported by both Netscape and IE, but not other browsers of the era - to Netscape only. Over time, Gecko, Konqueror, Opera, Safari and Chrome each decide to similarly spoof the User-Agent of some previous browser in order to manipulate browser-sniffing web pages into correctly understanding their browser's features. As part of this spoofing, all the browsers start their User-Agents with Mozilla/, like the browsers before them had done. Plenty of other nonsense also results, like modern Chrome's User-Agent simultaneously claiming to be Mozilla, Chrome, Safari, and 'like Gecko'.
其他回答
Mozilla/5.0是表示浏览器与Mozilla兼容的通用令牌，目前几乎所有浏览器都使用它。 更多详情请点击这里
原因是臭名昭著的浏览器大战。
简而言之，浏览器开始伪装成其他浏览器，因为一些网站根据浏览器的要求提供内容，他们也想要为其他浏览器提供内容。
令人惊讶的是，这个问题到今天还没有解决。既然userAgent由于遗留原因不能安全删除，为什么没有引入一些明确标识浏览器的新特性呢?是的，大多数时候你不需要浏览器检测，而是一个功能检测，但它也不是内置的JS !
正如@Jason所说，“说来话长”。总结一下这个故事“只是每个浏览器都假装是Mozilla”
在尝试了一切之后，我终于用到了:http://www.useragentstring.com/
您可以使用上述网站获得格式化的用户代理和操作系统。 他们有一个你可以直接使用的API…
这是一个漫长而悲伤的故事。
总而言之:
Mozilla browser gets released, with User-Agent Mozilla/1.0 (Win3.1). It is publicly renamed to Netscape, but in its User-Agent it keeps its original name . Internet Explorer is released. It spoofs Netscape by starting its User-Agent with Mozilla/ because web servers were routinely browser sniffing and serving pages with frames - a feature supported by both Netscape and IE, but not other browsers of the era - to Netscape only. Over time, Gecko, Konqueror, Opera, Safari and Chrome each decide to similarly spoof the User-Agent of some previous browser in order to manipulate browser-sniffing web pages into correctly understanding their browser's features. As part of this spoofing, all the browsers start their User-Agents with Mozilla/, like the browsers before them had done. Plenty of other nonsense also results, like modern Chrome's User-Agent simultaneously claiming to be Mozilla, Chrome, Safari, and 'like Gecko'.
因为这就是网景对自己的定位，而微软想要与那些能够检测网景并拒绝其他任何东西的网站合作。
