There are multiple levels to this, and it varies with chipsets. Take for example, a very basic crappy chipset like the TI WL127x transceiver series. This chipset would be the lowest level. It is what implements the lowest level of radio signal processing above the hardware. That chipset has its own firmware. That's very small. Typically, less than 128kbytes. But it is rarely factory set or read-only. Rather, it is a binary blob that is loaded on to that chipset by a linux kernel wifi driver. The driver is typically not a binary but source code that is part of the linux kernel. That blob, driver, kernel, and rest of the system are then part of a "firmware image" that is flashed on to your router/phone/embedded device. It is reasonably common to want to modify and customize that firmware image. Hence forums like xda, cyanogen, etc. The binary blob is the only part that isn't very common to modify, but even then, given the high rate of implementation defects in that blob, one wishes source were available for that as well so that it could be better controlled.