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.
If you would like to try out MP4 Downloader nightly builds based on the latest source code in the git repository, you can download the latest build from: http://mp4downloader.dyndns.org:1045/nightly/