Is it important for a web browser to have a sandbox? Or is it a superfluous feature?

Internet Explorer and Chrome/Chromium have a sandbox. I can't tell if Safari has one or not. Firefox does not have a sandbox.

I have read a couple of articles stating that Firefox (out of the box with no addons) is insecure because it lacks a sandbox (but then I read an article warning not to take those results too seriously).

Is a sandbox essential for security? Or is it better to say that it is very useful but can be replaced with addons like uMeme or NoScript?