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Normally Firefox captures disk space progressively as a file is downloaded. So it can come to a situation when you download 99% of a file to know it can't fit on your HDD or download 50% of 2 files and lose them both instead of saving 100% of one of them.

Can this behaviour be altered so that Firefox will write a full-sized dummy file on download start and replace its content with actual data as downloading? Some download managers have this in options, for example, but I prefer using standard Firefox built-in downlad manager.

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their could be an option in the firefox about:config page, be careful when editing this – Arch Angel Nov 5 '10 at 20:47
@arch-angel thank you, Captain Obvious :-) – Ivan Nov 5 '10 at 22:43

The keyword to your question is called 'pre-allocation'. firefox doesn't do this in recent versions as this causes a major freeze when downloading huge files (example, Firefox is frozen while making a 'dummy' 5Gb file. This freeze can last to even 10 minutes!)

Even though, I recommend you use the superb Firefox Addon: DownThemAll. It has the feature you need.

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