Create a firefox addon that change text on browser

I am responsible for creating and developing exciting graphics and visuals for client websites that make their pages beautiful and attractive. I have strong analytical skills that I regularly put into practice through heuristic testing and observation of user interfaces. In addition to my formal education and training, my 4+ year history of hands-on experiences as an Android Developer , Firefox, Java, Javascript, Windows Desktop make me an excellent match for the unique demands of this opportunity. Kindly reply me soon so we can discuss more about this. Thank you , The work provided by the costumer in detail will decide the first milestone.