javascript in Mozilla

A stupid question.
I have XUL window that uses .js file.
in .js file I have global variable.
When I open the window twice - what occures with my variable ? Will it 
be global in own window context only or it will be global for all 
windows using same .js file. What do I need to do to get variable global 
for all my project (firefox extension)?
Thanks.

Alex.