[keysnail] vimperator の C-^ みたいな

.keysnail.js

if (typeof gBrowser !== 'undefined' && gBrowser.tabContainer) {
    let prev = gBrowser.tabContainer.selectedIndex;
    let cur = prev;
    gBrowser.tabContainer.addEventListener("TabSelect", function(){
        prev = currentSelectedTabIndex;
        cur = gBrowser.tabContainer.selectedIndex;
    }, false);
    ext.add("toggle-selected-tab", function () gBrowser.tabContainer.selectedIndex = prev, '選択タブをトグルする');
}