Firefox Downloads

I'm wondering what the best rule is for firefox downloads.
Say, if I'm downloading a large .jpg file (4MB ish), and before that could finish downloading, Hazel would move the file to a folder I already designated for Picture files. Giving Firefox a download error.
What's the best solution to this?
Say, if I'm downloading a large .jpg file (4MB ish), and before that could finish downloading, Hazel would move the file to a folder I already designated for Picture files. Giving Firefox a download error.
What's the best solution to this?