Depends what you mean by the "HTML part". The HTML parser, which is the only HTML-specific part, was replaced in Firefox 4, with one implementing the algorithm

Depends what you mean by the "HTML part". The HTML parser, which is the only HTML-specific part, was replaced in Firefox 4, with one implementing the algorithm defined in the WHATWG HTML spec. The DOM code there's definitely less motivation to rewrite, in large part because there's a lot less benefit to be gotten from rewriting large parts of it (versus layout or style where there's much more entanglement across the codebase).