How secure is Safari 6.1.6 under Lion?

Hey there,

I'm still loving and using my old iMac from 2006. For my usage its still absolutely sufficient. However, since I can only use Lion I also can't update Safari anymore. The latest version I'm running is 6.1.6. My question is if it's still safe to use. I don't really like Firefox on OSX because the font's are too clumsy. So I don't really want to switch my browser.