Javascript + Firebug : "cannot access optimized closure" What does it mean?

Question

I just got the following error in a piece of javascript (in Firefox 3.5, with Firebug running)

cannot access optimized closure 

I know, superficially, what caused the error. I had a line

options.length()  

instead of

options.length 

Fixing this bug, made the message go away. But I'm curious. What does this mean? What is an optimized closure? Is optimizing an enclosure something that the javascript interpretter does automatically? What does it do?