Word document on Mac based browser

I am trying to develop a web based application to run on a Mac (as well as Windows). For cross platform compatibility I'm basing it on Firefox. The application must open (why? long story) a MS Word document on the IIS server for the user to make changes and save it back on the server. I used an HTML editor that somehow converts the Word document into HTML and presents it on the screen which kind of does the job but the customer is not happy due to some limitations (e.g. can't recognise header and footer). Recently I came across a product called Think Free which opens the actual document instead of converting it which is great but Think Free doesn't seem to work on Mac. Any suggestion how I can achive develop such application?