Graphene is right if you're afraid someone is trying to hack your phone with an RCE in some form of drive-by exploit to hijack your browser.

While RCE attacks and Firefox 0days do exist, I think the privacy improvements outweighs the anti-exploit benefits provided extra layers of sandboxing.

That said, Firefox does seem to be rolling out Fission on Android, which brings hope that site isolation may come soon so that we can have both benefits at the same time.