I've recently discovered 4K Video Downloader, which isn't a browser add-on, but it seems a decent and simple downloader app:
I've used it for a while but got annoyed by the ads trying to get me to buy the full version.
Then I discovered Free Download Manager
which has no ads and mostly does the job (though it seems to be limited to 1080p even when a 1440p version of the video is available).
It also allows you to change the bandwidth it uses for downloading on the fly, which may be useful if you have somewhat slow Internet access and decide you'd want to watch some streaming while you've already started a download.
Now since then I got fibre optics internet access at home, and don't need to limit my bandwidth usage any more, I've mostly switched to Youtube-DLG
(a minimalistic GUI for youtube-dl), which doesn't seem to allow changing the bandwidth on the fly, but does download 1440p videos when available.