This is horrible. A study has been carried out (it is worth noting that Google sponsored it, hence how Chrome obviously wins) that shows Chrome as the most secure web browser followed by Internet Explorer and then Firefox. The main reason that Chrome wins is probably because security (and speed) was one of the key aims when Chrome was originally created and every single tab, app and extension are sandboxed very tightly so that they can’t do anything that they shouldn’t be allowed to do. Internet Explorer has been slowly gaining this feature over the last few versions and it is still quite bad implemented.
I would say that Firefox has quite a lot of security though and it does sandbox to a certain extent, although probably not as much as the other browsers. Plugin security has been something that has been included in all three browsers for a while and theoretically Microsoft would win this if only Silverlight were judged.
I think that the main reason that Firefox has fallen behind is because the security was the main reason that people downloaded it initially. Aside from being faster, Firefox 1 was what was bringing people away from IE because it wasn’t going to infect their computer and had relatively good reviews. Over time Firefox added new problems that led to security holes.
By the time that Chrome was released Firefox was getting quite clunky and Chrome, being built from scratch, was offering both security and speed. Firefox then had to catch up with Chrome’s speed which meant that security was ignored and even now the new releases are advertised as being faster rather than safer. It doesn’t really surprise me that IE has improved in the last few years and the main reason is probably also Chrome; it needed to be better because it had already had such bad press.