Support for MP4 Downloader has been discontinued.
I don't have the time anymore to keep up with all the breaking changes that Mozilla is making in add-on development for Firefox, in addition to reacting to changes in YouTube, Dailymotion, and Vimeo that affect video downloading. The current version of MP4 Downloader has some features that are broken because of this, and once Mozilla drops traditional add-ons in favor of WebExtensions (something that I, and many others, don't agree with), MP4 Downloader will stop working entirely. (This change is planned for Firefox 57, which will be released in November 2017.)
The codebase has been left in the middle of a complete rewrite (see the Source Code section of the Development page). That codebase, and this website, are left here for archival purposes.
MP4 Downloader is a simple, cross-platform, open-source browser add-on that downloads videos from YouTube, Dailymotion, Facebook Video, and Vimeo as MP4 files. This add-on can currently run in Mozilla Firefox (as an add-on) or in any browser as a bookmarklet.
A good reason to download videos in the MP4 format is that this format is iPod-compatible without additional conversion. All videos downloaded using MP4 Downloader can be easily transferred to an iPod (or almost any other portable device). See the Sync to iPod page for details.