スレ違いだが、FireGestures で書くなら

var searchText = FireGestures.getSelectedText();
var ss = Cc["@mozilla.org/browser/search-service;1"].
            getService(Ci.nsIBrowserSearchService);
var isSearchbarVisible = isElementVisible(BrowserSearch.searchBar);
var engine = isSearchbarVisible ? ss.currentEngine : ss.defaultEngine;
var submission = engine.getSubmission(searchText, null);
if (!submission)
    return;
gBrowser.loadOneTab(submission.uri.spec, null, null, submission.postData, false, false);
if (searchText)
    BrowserSearch.searchBar.textbox._formHistSvc.addEntry("searchbar-history", searchText);