I think the safest browser would be the depending on what you do on the web, so it could be any (well the modern ones anyhow). If you are an addon fanatic like me, you have to be careful of addons in general and see what people in comments say about them. Addons give more freedom than the general Javascript on a webpage. Mainly Noscript and adblocker (which is basically on almost all modern browsers) are the best addons for security. The general answer is any browser would give you about the same line of security, it depends on what you do with your browser.