Basically it's small greasemonkey script which adds time and date to http://mini.pogoda.yandex.ru so it looks similar to Chrome Currently plug-in.
I'm using it as default page for new tabs in Firefox:
about:config
browser.newtab.url; http://mini.pogoda.yandex.ru/frankfurt%20am%20main
Screenshot Left side is Time4YaWeatherMini. Right side is Currently.