I'm not sure what supporting FireMonkey would deliver to the Lazarus project. There already is a multiplatform LCL, and Embarcadero hasn't really deprecated VCL yet.
Keep in mind that if you start, in OSS part time, with a totally new framework, it will take years to get on par with Embarcadero, and you don't want to be caught up in the "side" projects of Embacardero that are usually short lived (like .NET, CLX etc)