The browser is only as safe as the user who use adecuately all the possible security mechanismus of the browser!
Firefox can be very unsafe if someone (who doesnt know anything) does not configure it.
Opera performed better when tried/tested by visiting the "Darkest side of the Net":
i.e. heavily infected Russian, Chinese, XXX sites, etc.
And the rest of browsers: Crashed or frozen.
Special security add-ons as NoScript, AdBlock/Plus, etc. make Firefox the safest browser.
Under Linux and other GNU/Unix variants (all BSDs or perhaps OSX) and their browsers you can explore the web without the typical viral problems of the MS world!