Firefox OS is not a web browser but a mobile Operating System used on some phones and some recent models of Panasonic Tv's for example.

The Firefox shell browser for iOS devices needs at least iOS 8.2 at minimum. For the private browsing feature you need iOS 9.0 and newer.

Mozilla is making use of modern API's that are only available on newer versions of iOS.

No it cant run the list of firefox OS support mobile is simple and short. you can find it here............... https://firefoxos.mozilla.community/devices/