Cool trick of the day: "Uploading" URLs.

I know this works on Windows 7. I don't know about Vista or XP. I also don't know if the file dialog Firefox uses on Linux duplicates the behavior.

So you want to upload a file to some web form. Click Browse, that traditional file upload dialog pops up.

Into the "Filename" field, type (or paste) the full URL to the file you want to upload. For example, if the file already exists on Flickr or some other site, and you have the URL to the file at that location, paste it here.

Click Open; the dialog will appear to hang for a bit, but that's because it's downloading the file to your local system. When the dialog disappears, the path to its local location will appear in the upload field.

Proceed with however you would normally use the web form; the file was already copied to your local system, Firefox knows where it is, and it will be uploaded when you submit the form.