javascript in firefox extension to search

[Alex]

I think this is a general question about javascript, but i included
background in where i was using this code to give a clearer picture.

I creating a right-click tool, that allows a user to right-click on a
webpage, context menu shows up, click show vars, and a little msg box shows
up with the value of the variable.

I've got the firefox extension with the context menu, right-click thing
going, I'm new to javascripting. I just don't know how to search the page
source for text that looks like:
myVar = "some-data"

I want to be able to report "some-data" to the user using an alert(). if you
take a look at the source for this web page, it would look like:
.....
<!-- myVar = "some-data"; //-->
.....

any suggestions, code snippets, or links to tutorials for this stuff would
be appreciated.

can i start with var page = window._content.document; ? and somehow search
that page?

thanks