Hello all, Newbie needs some help...

I'm trying to write my first Firefox extension and I'd need some
guidelines here, I've been reading tutorials and googling around but
I'm a bit confused with all this stuff (haven't found a really
comprehensive tutorial). I'm somewhat familiar with javascript and my
"Hello World" extension is up and running.

Here's what I'm trying to do:

- when extension is started it opens a new tab (this one is already
figured out
- then it should log in to a site and read some HTML from there (I
quess XMLHttpRequest takes place here)
- modify HTML with javascript
- output modified page to newly opened tab

The questions are:

Am I going to run into difficulties when trying to retrieve data from
a site requiring username/password with XMLHttpRequest?

If you have a html page read into a javascript variable, how do you
output that content into a new tab?

Shed some light please. Also links to good examples and tutorials
would be highly appreciated.