SonyEricsson Hardware and Firmware Definitions! - Top Sony Forums

The firmwire is sorta like the OS, its made up of 2 parts FS and MAIN.
Firmware has two parts:
1. FS: It contains languages, images, sounds, animations, settings, etc. Anything that isn't pure software.
2. Main: It contains the software.

So the main is more to do with the OS.

SE makes the firmwires, but other companys insert all there logo's on to the firmwires, thats why we DEBrand our mobiles so its like the original Sony Ericsson phone how it came out of the company. - Thats one of the advantages of flashing a GENERIC firmwire.

Patches can only be installed on Debranded/Generic firmwires, patches are not created by SE but by users of Sony ericsson.