Please make Firefox tell Windows not to sleep when it is downloading files. Or perhaps, make this an optional behavior that can be toggled in settings.
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This should be a very basic feature. The only current ways to handle this is to either turn off sleep completely (very wasteful as the download might be done way before you come back to turn off the computer or change the setting back, and also very inconvenient), or to use a third party program that might or might not work.