Is this for a specific file or just in general? As far as I know, you cannot view word files in Firefox without downloading them. However, there is a built-in pdf viewer.

Any file that you want to view need to be "downloaded" either to your hard drive or to the "temporary" or TEMP folder [as named on Windows OS's].

The exception would be "web-type" files such as HTML and maybe XUL.

You could check with Apple or with Microsoft to see if there is a "Word Plugin" for Firefox that would help you "streamline" the viewing of those files.

I don't use a Mac and I don't use Word, but Libre Office and Open Office programs do come with a "browser Plugin" for Firefox that allows certain open format files to be viewed in Firefox; such files are saved in the cache folders of Firefox, but they can't be edited All the user needs to do is to enable that plugin in OOO or Libre Office preferences.