Developer Needed! - Extension for transfering data between different windows

I have been looking for a way to transfer data between form fields on
two different windows without using plain old copy/paste field by
field which is just tedious. We have to transfer address information
from our site (which we can code at will) to another company's site
(which we have no control over) all day long.

I haven't been able to find a method purely in JavaScript that can do
this so far but I was wondering if anyone knew of a way to do it
through a Firefox extension we could develop. I would like to
determine if there is some way we could automate such form data
transfers.

I thought about using GreaseMonkey for this instead of developing an
entire extension just for the purpose but I haven't a clue how to move
the data between the windows with that either.

This has become an issue that we need resolved sooner than later so we
would be willing to pay a developer to create such a GreaseMonkey
script or even a full extension if that is what is needed. Email me
with pricing/rates (or respond to this post) if you can implement the
functionality we need.