Firefox downloads video as a Textedit file.
When my hard drive died I replaced it with a new one and got Firefox and Downloadhelper to take care of downloading. Never had any trouble with that before. Now when I download a video it goes to my movies folder as a Textedit file. No video but hundreds or thousands of pages of squiggles.
iMac (24-inch Early 2008), Mac OS X (10.7.5)