The work done to incorporate PDF support into Firefox is great. It is functional, not clunky like other solutions and for the majority of people means they no longer require a 3rd party PDF software.
The djvu format is also widely used and in many ways a better format (although both have their strong points and fit slightly different use cases), however there is little support outside specialist software for the djvu format despite it being a better format in many cases. It would be brilliant if the djvu format could be incorporated into the Firefox PDF interface as another format option.