Start an activity from a fragment

If you are using getActivity() then you have to make sure that the calling activity is added already. If activity has not been added in such case so you may get null when you call getActivity()

in such cases getContext() is safe

then the code for starting the activity will be slightly changed like,

Intent intent = new Intent(getContext(), mFragmentFavorite.class);
startActivity(intent);

Activity, Service and Application extends ContextWrapper class so you can use this or getContext() or getApplicationContext() in the place of first argument.

mFragmentFavorite in your code is a FragmentActivity which is not the same thing as a Fragment. That's why you're getting the type mismatch. Also, you should never call new on an Activity as that is not the proper way to start one.

If you want to start a new instance of mFragmentFavorite, you can do so via an Intent.

From a Fragment:

Intent intent = new Intent(getActivity(), mFragmentFavorite.class);
startActivity(intent);

From an Activity

Intent intent = new Intent(this, mFragmentFavorite.class);
startActivity(intent);

If you want to start aFavorite instead of mFragmentFavorite then you only need to change out their names in the created Intent.

Also, I recommend changing your class names to be more accurate. Calling something mFragmentFavorite is improper in that it's not a Fragment at all. Also, class declarations in Java typically start with a capital letter. You'd do well to name your class something like FavoriteActivity to be more accurate and conform to the language conventions. You will also need to rename the file to FavoriteActivity.java if you choose to do this since Java requires class names match the file name.

UPDATE

Also, it looks like you actually meant formFragmentFavorite to be a Fragment instead of a FragmentActivity based on your use of onCreateView. If you want mFragmentFavorite to be a Fragment then change the following line of code:

public class mFragmentFavorite extends FragmentActivity{

Make this instead read:

public class mFragmentFavorite extends Fragment {

I use this in my fragment.

Button btn1 = (Button) thisLayout
            .findViewById(R.id.btnDb1);

    btn1.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            Intent intent = new Intent(getActivity(), otherActivity.class);
            ((MainActivity) getActivity()).startActivity(intent);

        }
    });

    return thisLayout;
}

You should use getActivity() to launch activities from fragments

Intent intent = new Intent(getActivity(), mFragmentFavorite.class);
startActivity(intent);

Also, you should be naming classes with caps: MFragmentActivity instead of mFragmentActivity.