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. Since Mozilla dropped support for traditional add-ons in November 2017, MP4 Downloader has stopped working entirely in new versions of Firefox.
The codebase has been left in the middle of a complete rewrite (see the Source Code section of the Development page), although this rewrite is not anywhere close to what is required to support new versions of Firefox. That codebase, and this website, are left here for archival purposes.
To all MP4 Downloader fans, thanks for many years of support.
MP4 Downloader operates a beta channel for testing purposes. If you would like to help the development of MP4 Downloader, you can download the latest beta version, test it, and provide feedback.
You can download the latest beta version from Mozilla Add-ons (scroll down to the bottom of the page and click "Development Channel").
If you're having problems with AMO, try this: Direct Installation Link (version 1.3.3b3)