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.