Firefox: show download rate in built-in download panel, code for Custom Buttons extension https://addons.mozilla.org/addon/custom-buttons/ ("initialization" section) http://custombuttons.sf.net/forum/viewtopic.php?t=1006

showDownloadRate.js

// https://gist.github.com/Infocatcher/5787749
// More: https://github.com/Infocatcher/Download_Panel_Tweaker
// (c) Infocatcher 2013

function showDownloadRate(patch) {
	//var {DownloadUtils} = Components.utils.import("resource://gre/modules/DownloadUtils.jsm", {});
	const bakKey = "_customButtons_getDownloadStatusNoRate";
	if(!patch ^ bakKey in DownloadUtils)
		return;
	if(patch) {
		DownloadUtils[bakKey] = DownloadUtils.getDownloadStatusNoRate;
		DownloadUtils.getDownloadStatusNoRate = DownloadUtils.getDownloadStatus;
	}
	else {
		DownloadUtils.getDownloadStatusNoRate = DownloadUtils[bakKey];
		delete DownloadUtils[bakKey];
	}
}

function destructor(reason) {
	if(reason == "update" || reason == "delete")
		showDownloadRate(false);
}
if(
	typeof addDestructor == "function" // Custom Buttons 0.0.5.6pre4+
	&& addDestructor != ("addDestructor" in window && window.addDestructor)
)
	addDestructor(destructor, this);
else
	this.onDestroy = destructor;

showDownloadRate(true);

The same without undo ability (and not only for Custom Buttons):

DownloadUtils.getDownloadStatusNoRate = DownloadUtils.getDownloadStatus;