Tricky way to detect if there something to remove using nsIDownloadHistory.removeAllDownloads()

getDownloadHistory.js

function getDownloadHistory() {
	var query = PlacesUtils.history.getNewQuery();
	query.setTransitions([Components.interfaces.nsINavHistoryService.TRANSITION_DOWNLOAD], 1);
	var options = PlacesUtils.history.getNewQueryOptions();
	options.resultType = options.RESULTS_AS_URI;
	options.queryType = Components.interfaces.nsINavHistoryQueryOptions.QUERY_TYPE_HISTORY;
	options.includeHidden = true;

	var result = PlacesUtils.history.executeQuery(query, options);
	var contents = result.root;
	contents.containerOpen = true;

	var uris = [];
	for(var i = 0, l = contents.childCount; i < l; ++i)
		uris.push(contents.getChild(i).uri);
	return uris;
}

function hasDownloadHistory() {
	var query = PlacesUtils.history.getNewQuery();
	query.setTransitions([Components.interfaces.nsINavHistoryService.TRANSITION_DOWNLOAD], 1);
	var options = PlacesUtils.history.getNewQueryOptions();
	options.resultType = options.RESULTS_AS_URI;
	options.queryType = Components.interfaces.nsINavHistoryQueryOptions.QUERY_TYPE_HISTORY;
	options.includeHidden = true;
	options.maxResults = 1;

	var result = PlacesUtils.history.executeQuery(query, options);
	var contents = result.root;
	contents.containerOpen = true;

	return !!contents.childCount;
}

alert(hasDownloadHistory() + "\n" + getDownloadHistory().join("\n"));

hasDownloadHistory.js

function hasDlHistory() {
	try {
		var stmt = PlacesUtils.history.DBConnection.createStatement(
			'select place_id from "moz_annos" where anno_attribute_id=\
				(select id from "moz_anno_attributes" where name="downloads/destinationFileURI")'
		);
		var ids = [];
		while(stmt.executeStep())
			ids.push(stmt.row.place_id);
		Services.console.logStringMessage("hasDlHistory(): ids:\n" + ids.join(", "));
		if(!ids.length)
			return false;
	}
	catch(e) {
		Components.utils.reportError(e);
	}
	finally {
		stmt && stmt.finalize();
	}

	if(ids) try {
		var stmt = PlacesUtils.history.DBConnection.createStatement(
			'select url from "moz_places" where id = ' + ids.join(" or id = ") + " limit 1"
		);
		if(stmt.executeStep()) {
			Services.console.logStringMessage("hasDlHistory(): url: " + stmt.row.url);
			return true;
		}
		Services.console.logStringMessage("hasDlHistory(): Not found in moz_places");
		return false;
	}
	catch(e) {
		Components.utils.reportError(e);
	}
	finally {
		stmt && stmt.finalize();
	}

	return undefined;
}
alert(hasDlHistory());

hasDownloadHistoryAsync.js

function hasDlHistoryAsync(callback, context) {
	var stmt = PlacesUtils.history.DBConnection.createAsyncStatement(
		'select place_id from "moz_annos" where anno_attribute_id=\
			(select id from "moz_anno_attributes" where name="downloads/destinationFileURI")'
	);
	var hasRes;
	var ids = [];
	stmt.executeAsync({
		handleResult: function(res) {
			hasRes = true;
			for(var row; row = res.getNextRow(); )
				ids.push(row.getResultByName("place_id"));
		},
		handleCompletion: function(reason) {
			var failed = reason != Components.interfaces.mozIStorageStatementCallback.REASON_FINISHED;
			if(!hasRes || failed) {
				callback.call(context, failed ? undefined : false);
				return;
			}
			Services.console.logStringMessage("hasDlHistoryAsync(): ids:\n" + ids.join(", "));
			if(!ids.length) {
				callback.call(context, false);
				return;
			}

			var stmt = PlacesUtils.history.DBConnection.createAsyncStatement(
				'select url from "moz_places" where id = ' + ids.join(" or id = ") + " limit 1"
			);
			hasRes = false;
			stmt.executeAsync({
				handleResult: function(res) {
					hasRes = true;
					var row = res.getNextRow();
					callback.call(context, !!row);
					Services.console.logStringMessage("hasDlHistoryAsync(): url: " + row.getResultByName("url"));
				},
				handleCompletion: function(reason) {
					var failed = reason != Components.interfaces.mozIStorageStatementCallback.REASON_FINISHED;
					if(!hasRes || failed)
						callback.call(context, failed ? undefined : false);
				}
			});
		}
	});
	stmt.finalize();
}
hasDlHistoryAsync(function(has) {
	alert(has);
});