[keysnail] vimperator の C-^ みたいな

.keysnail.js

if (typeof gBrowser !== 'undefined') {
    let previousSelectedTabIndex = gBrowser.tabContainer.selectedIndex;
    let currentSelectedTabIndex = previousSelectedTabIndex;
    gBrowser.tabContainer.addEventListener("TabSelect", function(){
        previousSelectedTabIndex = currentSelectedTabIndex;
        currentSelectedTabIndex = gBrowser.tabContainer.selectedIndex;
    }, false);
    key.setGlobalKey('C-^', function (ev, arg) {
       gBrowser.tabContainer.selectedIndex = previousSelectedTabIndex;
    });
}