Possibly for the same reason they don't ship Tk compiled versions. Why should they target every GUI toolkit and complicate their build process which they then have to deal with supporting?
If it's that easy, you can probably find some third party that compiles and offers it. I'm not sure why you think Firefox needs to do it themselves.
If it's that easy, you can probably find some third party that compiles and offers it. I'm not sure why you think Firefox needs to do it themselves.