Firefox doesn't natively decode Word documents into HTML; you would need a viewer plugin. I don't know whether Microsoft has released one for browsers other than IE.

Searching the web, there have been some attempts at this in the past, but most seem to have faded away... if you have time to chase leads, please have at it.

let me be a little bit clearer. I want to open an office doc from a Sharepoint site using firefox. I also changed the settings of the Sharepoint site so it will open document on the browser as a default. the problem is : when I try to open an office doc automatically while using FF it tries to open it on the client computer BUT when I try to do the same thing manually it opens in the browser.

Discussion continues here: https://support.mozilla.org/en-US/questions/933236