Modify source code of Mozilla Firefox browser

Want to change source code of Firefox browser as per my requirement.Don't want/removing function's like New window, New private window, New Tab button,Bookmark, History, Bookmark etc. is that possible?

But the security should remain as it is.

Want to use the same as the front end of my software.

Also would like to know that is it possible to link the modified Firefox browser to local mySQL which later get synced with the cloud database when Internet connection is available?

Where to find the source code of Firefox Browser as have tried downloading the same from Mozilla Central with bz2 and ZIP file type but due to patchy internet connectivity it gets failed so want a reliable source with resume capability tried downloading using download manager.