Fix issue with ElasticWolf opening on Firefox start-up.

> Ideally EW should not auto-start when Firefox starts

The offending line in _prefs.js_:

    pref("toolkit.defaultChromeURI", "chrome://ew/content/ew_window.xul");

This line is needed for XULRunner: otherwise, the main window will not open.

I've created two preference files, one for XULRunner and one for Firefox. The latter simply omits the above line. The appropriate preference file is linked at build time.

I've also expanded the Makefile so that the Firefox extension can be installed by running

    make install

## Testing

On Mac:
* `make install` and open in Firefox
* Open with `make run`
* Decompress the OS X zip and run