いろいろ試したけどダメだった。
誰か助けて(TT)

var url = window.content.location;
var txt = '&lr=lang_ja'
gBrowser.loadURI(.append(url[,'txt']));

var url = window.content.location.href.append("&lr=lang_ja");
gBrowser.loadURI(url);

var url = window.content.location;
var txt = '&lr=lang_ja'
gBrowser.loadURI(url + txt);

var url = window.content.location.href + '&lr=lang_ja';
gBrowser.loadURI(url);

var url = window.content.location + '&lr=lang_ja';
gBrowser.loadURI(url);