History navigation and DOM + JS context

An observation I made with FF 3.0.10 is that history navigation sometimes
results in a JavaScript re-initialization but other times it does not. I
think it depends on the complexity of the script but I do not have any
concrete answer at this point. It seems like FF is making some determination
on when it is worth it to cache the DOM / JS Context vs. when to start over
again.

What determines this behavior?

(FWIW: In IE and Chrome, the JS / DOM is re-initialized on each history
navigation)

Thanks,
Manoj