There is a standalone Firewing executable supplied with the IDE, which you should be able to integrate into MPLAB - in a similar way you would Swordfish:
http://www.sfcompiler.co.uk/wiki/pmwiki ... User.MPLABFirewing can also generate a COFF file. Early in development, I had Firewing simulating happily in MPLAB, but please note this is something a won't be supporting initially as I don't currently have the resources to do so. However, if you get it all up and running, please do post your findings. If you get stuck let me know and I will help as best I can, time permitting.