I have no idea how this app works. It has been on my computer for a few years but I could never get it to work, although I kept trying intermittently. About a month ago it worked for the first time and now it's working all the time - but I still don't know how or why. Documentation might help.

bisher alles in Ordnung, seit kurzem wird ein QR-Code sehr groß auf das Video gelegt. Das ist sehr ärgerlich

Downloads Youtube videos as webm which is fine. Not so fine is the QR code watermark stamped in that needs a paid licence to remove. That info should be upfront, before download and install, not afterwards. A pity because I like their ethos, their transparency around donation problems they found using Paypal (tl;dr use stripe.com, not Paypal) and their support for Kiva donations. As they say, they need to eat too. Just let us know about QR codes first :)

jó program, de nincs magyar nyelv és sokszor nem értem, hogy mit akar, emiatt idegesítő volt a használata

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.

ottimo