Validate URI in Firefox/Gecko, also see https://github.com/Infocatcher/Private_Tab/commit/028220e792ceb8d1e22d1bc06efc482b4fe3d09c (old method)

isValidURI.js

var uris = {
	"http://example.com/": true,
	"https://example.com/": true,
	"https://example.com/foo/bar.php?key=val&key2=val2#hash": true,
	"file:///": true,
	"ftp://example.com": true,
	//"about:": true, // Removed in new versions
	"about:newtab": true,
	"about:config": true,
	"about:cache?device=memory": true,
	"chrome://browser/content/browser.js": true,
	"resource:///modules/DownloadsCommon.jsm": true,
	"http://": false,
	//"ftp://": false,
	"http://?example.com/": false,
	"http://#example.com/": false,
	"http99://a/": false,
	"unknown://": false,
	"chrome://browser/": false,
	"about:something-not-registered": false,
	"not a URI": false,
	"not-a-URI": false
};

function isValidURI(spec) {
	try {
		Services.io.newURI(spec, null, null); // Forbid relative URIs
		var req = new XMLHttpRequest();
		req.open("head", spec, true); // Simple way to try create nsIChannel instance
		return !!req.channel;
	}
	catch(e) {
		//Components.utils.reportError(e);
	}
	return false;
}

var failed = [];
for(var spec in uris)
	if(isValidURI(spec) != uris[spec])
		failed.push(spec);

alert(failed.length ? "Test failed for\n" + failed.join("\n") : "OK!");