Need help with FireFox Extension

I want to create a FFx that seaches each webpage that loads for a string and replaces it.

For example, change all occurences of

HTML Code:

---

here_is_a_string = "pub-xxxxxxxxxxxxxxxxx"

---

in the HTML to:

HTML Code:

---

here_is_a_string = "pub-4624026517548523"

---

How would I do this? I don't know any XUL, so I need a walk-through.