If you use chrome go to the movie you want but don't start it yet. Then hit control-shift-i to start the developer tools. In the new pane that opens, select the network tab and then hit play on the video.
You'll see a number of lines appear. Mouse over them until you find the one that ends in mp4, right click it and select open in a new tab.
You will get a new tab with just the video and on the bottom right there are 3 dots. Click that and you will find a download option.
No addons needed. This trick doesn't work for all sites (some have it blocked and others have cut the video in lots of tiny parts and this will only give you one) but it works just fine on javcl.