Reading the Javascript API docs here ...
https://www.zotero.org/support/dev/client_coding/javascript_api
there is reference to Plain Old Webserver.

But on trying to install POW I see that POW is incompatible with Firefox 40. What should I use instead of POW to get collectionID?

From the docs ..

Get the items for a particular collection

var Zotero = Components.classes["@zotero.org/Zotero;1"] .getService(Components.interfaces.nsISupports).wrappedJSObject;
var collectionid = pow_server.GET.id; // or some other way of finding the collectionID here
var collection = z.Collections.get(collectionid);
var items = collection.getChildItems();
// or you can obtain an array of itemIDs instead:
var itemids = collection.getChildItems(true);