正如 Jamie Wong 所说，Greasemonkey 脚本（或 JavaScript）都无法告诉客户端文件下载的状态。
但是，如果您的 JavaScript 是从您控制的服务器加载的，您可以设置一个系统来轮询服务器有关下载状态的信息。
但是，是的，您可以编写一个可以查看文件下载状态的 Firefox 扩展。......
至于在GM中使用jQuery，您可以在 这个 stackoverflow 答案。
As Jamie Wong said, neither a Greasemonkey script (nor JavaScript) can tell the status of a file-download, client-side.
However, if your JavaScript is loading from a server you control, you can set up a system to poll the server about download status.
But, yes, you could write a Firefox extension that can see file download status.......
As for using jQuery in GM,you can see a simple Greasemonkey script, that uses jQuery, in this stackoverflow answer.
允许 Javascript 访问浏览器的本机功能（例如下载）将是一个安全漏洞，因此是不可能的。
Allowing Javascript to have access to native functionality of your browser (such as downloading) would be a security hole, and is therefore not possible.
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正如 Jamie Wong 所说，Greasemonkey 脚本（或 JavaScript）都无法告诉客户端文件下载的状态。
但是，如果您的 JavaScript 是从您控制的服务器加载的，您可以设置一个系统来轮询服务器有关下载状态的信息。
但是，是的，您可以编写一个可以查看文件下载状态的 Firefox 扩展。
......
至于在GM中使用jQuery，
您可以在 这个 stackoverflow 答案。
As Jamie Wong said, neither a Greasemonkey script (nor JavaScript) can tell the status of a file-download, client-side.
However, if your JavaScript is loading from a server you control, you can set up a system to poll the server about download status.
But, yes, you could write a Firefox extension that can see file download status.
......
As for using jQuery in GM,
you can see a simple Greasemonkey script, that uses jQuery, in this stackoverflow answer.
允许 Javascript 访问浏览器的本机功能（例如下载）将是一个安全漏洞，因此是不可能的。
Allowing Javascript to have access to native functionality of your browser (such as downloading) would be a security hole, and is therefore not possible.