How to access webpage data from a firefox extension?
I have managed to get a custom very basic extension running in Firefox. 

What I want to do next is:

 1. Check if the user is on a desired webpage
 2. If yes: check the page on each postback for a hidden element containing a *string*
 3. If found: launch an external app with *string* parameter

I have some experience with javascript on webpages, but I don't know how to register my script to run on each webpage opened in firefox and how to access elements within a page.

Hints on where to start would be appreciated...
How to access webpage data from a firefox extension?
===

I have managed to get a custom very basic extension running in Firefox. 

What I want to do next is:

 1. Check if the user is on a desired webpage
 2. If yes: check the page on each postback for a hidden element containing a *string*
 3. If found: launch an external app with *string* parameter

I have some experience with javascript on webpages, but I don't know how to register my script to run on each webpage opened in firefox and how to access elements within a page.

Hints on where to start would be appreciated...