The default setting "/Applications/Firefox.app/Contents/MacOS/firefox-bin" tries to start a new instance of Firefox every time, which is refused blatantly by Firefox if there is already one.
Then I tried to use the "open" magic and set it to:
Name: Firefox
Location: /usr/bin/open
Parameters: -a /Applications/Firefox.app %URL%
which solved the multiple instances problem.
However, another problem popped up. The URL of API gotten from the code under cursor(Open External Javadoc) lost its anchor part, i.e., ".../docs/reference/android/widget/ImageView.html#setImageDrawable(android.graphics.drawable.Drawable)" became ".../docs/reference/android/widget/ImageView.html". So after opening the page, I have to locate the API by myself, which is really tedious and unproductive.
So, how can I properly set Firefox as the External Web Browser to overcome both problems together?