Actually, it is still true. Electroloysis split up Firefox into multiple processes, but not every site gets its own process. Work is underway to change this and achieve full site isolation in https://wiki.mozilla.org/Project_Fission.
(Disclosure: I work for Mozilla, but not on this)
Afaik, it uses Chromium's sandboxing?
That said, safety is about much more than just sandboxing.