Step:1 - Start
Step:2 - Draw a rectangale in Flash Scene
Step:3 - Select the Rectangle. Right Click the mouse then convert it to button or movie Clip.
Step:4 - Select the movieClip or button. then press F9 for Writing below code.
on(release){
getURL("file_name.htm","_blank"); // file name with path if html file is located in different folder.
}
Step:5 - Press Ctrl+Enter to Test movie.
Step:6 - Press the button.
Step:7 - This will open that html file in Browser window.
Step:8 - End.

Create a MC and name it e.g. my_mc
my_mc.onRelease = function(){
getURL("somePage.htm","_blank");
}