"Current free browsers do not offer a facility to run your own modified version instead of the one delivered in the page."

GreaseMonkey can do that in Chromium [http://dev.chromium.org/developers/design-documents/user-scr...]. Stallman mentions Greasefire later on, but it sounds like he might not clearly understand that Greasefire finds applicable GreaseMonkey scripts, while GreaseMonkey enables user scripting. He may not realize it, but I think the infrastructure is already mostly available for building the detect-and-substitute feature he wants.