OK, I ran into this problem today and luckily Googling answered my question. I thought it would be nice to share :)
I was working on a system where I had to open a new window, where the user would select a certain item from a list, and when clicking on that item, the item would be selected in the original window. Problem: I got an error message:
Error: uncaught exception: Permission denied to get property Window.miloform
Now, thanks to the Gecko DOM reference I found the answer to my problem:
It turns out that the client I am building this for has two domain names, and the new window was opened from a different domain. Yes, that would be a problem, knowing the above.