For "large" projects you do as every other language does: you link/FFI to the C library that does the heavy lifting for you.
It's kind of unfair to say "forth can't do browsers" when all browsers are implemented in C/C++ at the core.
You link to C when you need speed, and that's not even usually necessary anymore in languages like JavaScript or Lisp. Linking to C doesn't let your code somehow abstract higher, to deal with larger projects and larger architectural scopes.
