I just spent a couple of hours trying to figure out how to open new tabs from a Firefox sidebar. I found lots of different suggestions but none worked. Well finally I found this and it works.

var browser = top.document.getElementById("content");
var tab = browser.addTab("http://www.google.com");

An even better way of doing it is through:

const kWindowMediatorContractID = "@mozilla.org/appshell/window-mediator;1";
const kWindowMediatorIID = Components.interfaces.nsIWindowMediator;
const kWindowMediator = Components.classes[kWindowMediatorContractID].getService(kWindowMediatorIID);
var browserWindow = kWindowMediator.getMostRecentWindow("navigator:browser");
var browser = browserWindow.getBrowser();
var tab = browser.addTab("http://www.google.com");

This will work from anywhere in Firefox. Sidebar, popup and ordinary pages.

got from Larry William's Weblog

XS: IE and Mozilla also provide the _search value to open links in the search sidebar.

you got to put quote tags around that, since you have got it from another site!