VideoDownloadHelper is, conceptually, a great add-on for Firefox. I feel, however, that the product (sadly) still has some further thought needing to be put behind it to turn it into the kind of download helper that it could, and, should be.
When it works, it usually works well. Sometimes, however, for no reason, it does not work.
My biggest gripe, actually, is that the functionality is very esoteric / arcane in that the developer, even with the right intentions, has not "translated" the functionality into something that is easy to use / intuitive / friendly. Frankly, I find this a bit odd -- how does one not find out from potential users what their issues are, etc.?
There are still quite a few aspects to this add-on that, frankly, I have no clue about, and, can't make sense of. And, by the by, I'm an above-average tech user, so, I SHOULD be able to follow the logic easily.
So, so ... not meaning to be harsh. This product, 70% of the time, is very useful. I urge the developer, though, to elevate this app to a "100-percenter" which would be amazing, and, which would become a major money-spinner for said developer. Right now, it's a half-job, sadly, though, still worth going through the motions to use it, as, often, it does work well. I think (maybe?) a first step for the dev. should be to make usage of the add-on a LOT clearer and easier. I mean, this is overly obvious. From there, some good tweaking / researching users would / could make this a fine add-on.
My humble 5c worth...

Agree - It can make firefox very slow and unstable, especially when there are live videos, it starts to consume all the ram

Works here and there. Used it for a long time and it was quite good back in the day, however these days I run into more and more websites that give me server errors as the download starts.

Для любого действия, даже если не нужна конвертация, требует установить доп. приложение. Нафиг такое расширение.

Works on almost everything but regularly double, triples or quadruples videos. Like one 25min video will be a 50min long video after download because the original was downloaded twice on the same file.

superb downloader! works on almost **all** sites. love it. it's my primary downloader.
2022.jan: not working in 95.0.2 for facebook
2022.jan: does not download vk.com

ottimo

very slow

Not perfect, but reliable and fast. Only problem is it recently limits the number of downloads you can get

Works half of the time. In the other time, it forrces you to install a companion.

Сначала он загружает видео в течение 2 минут, а затем еще 10 минут он принудительно конвертирует его с высокой загрузкой процессора, накладывая в углу бесполезный QR-код. А потом, чтобы убрать этот QR-код, который они только что наложили за счет моего времени, и нагрузки на мой компьютер, "предлагают" заплатить $29.

Eng.:
First it downloads the video for a 2 of minutes, and then for another 10 minutes he forcibly converts it with high CPU load, overlaying a useless QR code in the corner. And then to remove this QR code they "offer" to pay $29.