There's a stack of tools around.
Try Video DownloadHelper plug for firefox first. If that doesn't work try something like Replay Media Catcher.
Just a shame that the 'standard' add-on doesn't have the ability to download 1080p (or higher clips), only max. 720p, and even then only if its the 'right' format. We used to be able to download 1080p on YT etc, not any more (I think they change the format of the files deliberately so you can't). Supposedly you can install a 'program' (not another add-on) to help Video DownloadHelper but I never like doing that as it could eaily be a way of getting computer viruses or facilitate hacking/phishing etc - anyone tried to use the program Video DownloadHelper recommends to facilitate 1080p+ downloads?
I've downloaded 1080P stuff with video downloadhelper before. It doesn't always pick up the full HD files though and I don't know why.
If you're downloading youtube clips, best tools i've used are jdownloader and 4kvideodownloader - both are free.
So have I - it could be because YouTube changed the file format of its files (some of their 4K videos previously downloaded before this problem never really run well on WMP or VLC and not at all on my tablet), and/or perhaps some incompatabilities with the add-on program with newer versions of Firefox - Video downloadhelper is now classed as a 'legacy' add-on, along with the REALLY USEFUL DownThemAll! (and its related 'anti-container' add-on) multi-download add-on, which means if they don't get updated to fit in with Firefox's new software format in version 57 (its been known since v55 a few months ago), then they won't work at all. Many of these indepenednt software developers aren't happy (especially the chap who designed DownThemAll!) with the changes that FIrefox are making and may not bother to keep updating theirs. If a large number of similar developers think that way, Firefox could easily go the way of the dinosaurs! That would be a real shame, given its by far the best web browser for useful add-on functions and is still reasonably swift in comparsion to the far lighter rivals, who aren't as good at blocking ads & pop-ups or privacy (bespoke tracking), in my view.
I'll give the 4K downloader a go.