Is there any way to detect whether a download has completed or is how much it has downloaded , in the context of a greasemonkey script Or and extension .(i.e) how to detect if firefox is still downloading a file or not, in a greasemonkey script or an extension ?
Also please give a Reliable way for using jQuery in greasemonkey.
Allowing Javascript to have access to native functionality of your browser (such as downloading) would be a security hole, and is therefore not possible.
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.
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As for using jQuery in GM, you can see a simple Greasemonkey script, that uses jQuery, in this stackoverflow answer.