With Firefox OS, they will be using web standard APIs. For things that do not yet have official APIs, they will make them and try to get them standardised.[0] This is unlike PhoneGap, which uses non-standard PhoneGap-specific APIs. This means that Firefox OS apps should also work fine as vanilla web apps in newer browsers.
That isnt quite how I would describe it, FirefoxOS is the realisation of Phonegap original vision. Phonegap was built to not exist, it patches device API's where they dont exist with the intention that when the API is supported natively phonegap just isnt needed.
Ideally FirefoxOS and Phonegap applications will be identical (in that they will all be web apps that use standard APIs) there is work being done on making that true.
[0] https://wiki.mozilla.org/WebAPI/