Without meaning to sound flippant, the only way is to write a Clutter backend for gecko. It has been done before in the past for Moblin (not sure where the code for it is anymore though) but it's a lot of work.
Basically, Mozilla don't seem to want you embedding Gecko into things, which is why they no longer support gtkmozembed. Porting the page to work with webkit would be a lot simpler.