As Mr. Patrick Walton over there said, Mozilla is still building up the exact, minimal file-system permissions. So, yes, in that way it is still not yet on quite the same level as Chrome.

In terms of security architecture, there is to my knowledge only one bigger difference left and you can change that, if you want.

The difference is that Chrome will spawn a new process for every new tab (unless the webpage in it is from the same domain as another tab). Firefox instead will always round-robin the tabs across a fixed number of processes to achieve lower RAM usage and as result of that also somewhat better performance.

But you can tell Firefox to round-robin across up to 1000 processes or what have you, so that it then does spawn a new process for every new tab (and therefore sandboxes each tab individually).

To do so, go into about:config and set "dom.ipc.processCount" to a high number, like 1000.