I am regularily using Download Helper to download video, to watch them later when in transit. The best part of this plugin is that it reliably detect any video or audio on the page, and allow the downloading of it.
But I am now searching for a alternative, because they are pushing harder and harder to install their "ConvertHelper" (FFMpeg rebranded), which I don't trust at all, is now "neccessary" to download video that were easily downloadable without some month ago, and is a paying, closed source product. (And let's not talk about the ethics of selling a open source product)
I know there is a lot of "Youtube downloader" addons for firefox, but I also grab video that are on independent site, and as such need the plugin to detect automatically the source.
A Firefox add on is prefered, but I am open to reputable / open source executable on Windows.