Firefox still doesn't use process separation between page rendering and the browser chrome. The thing that renders the pages on Chrome is a subprocess per tab (at considerable memory cost) which is also running in a sandbox.
In Firefox all tabs run in the same process and thus inherently can't be sandboxed (because it needs to write to the disk cache and save files the user downloads)
In Firefox all tabs run in the same process and thus inherently can't be sandboxed (because it needs to write to the disk cache and save files the user downloads)