Xfinity modems are DocSIS based devices. Verizon FiOS is MoCA based. Completely different methodolgies and frequencies. You must use the ActionTEC router if you're on FiOS via Coax since you need the dual frequency (WAN/LAN) capability for injecting the local network onto the Coax so that the STB's get get the guide data and such.
If you know what you're doing, you could have FiOS switched over to Ethernet handoff from the ONT and use a standard residential router (Linksys, Belkin, Netgear, etc.) for internet connectivity. However, if you have TV with STB's, you will still need to bridge the local LAN onto the Coax with a MoCA bridge so those devices can function. In such a scenario however, you probably want a MoCA low-pass filter at the point of entry of your Fios coax in order to filter off any FiOS MoCA.
Such a setup is not a trivial setup to do properly however.