Firefox extension to modify HTML

Question

I want to create a Firefox extension that creates a toolbar button with 2 options - on and off.

When On is selected, I want to check if the user is on a specific domain(s) (e.g. www.xyz.com/page1.html) and if he is, I want to edit html of that page and add insert a Javascript entry in that page. I also want to change all the textarea tag definitions on that page to call some javascript functions that are defined in the javascript that got inserted.

How can I do this?

Answer 1

Yes: Greasemonkey. (wikipedia)