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Из ВК скачивает файл, не являющийся видео, и даже примерно не равный ему по размеру. Firefox 67.0b7 x64, Windows 10.

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Installed Video Downloader Helper which wanted me to install an external program so uninstalled that straight away. Came across this add-on which does the same sort of thing BUT uses Firefox's own internal downloader. Although I've only used it once it worked perfectly. It's free, simple to use, lots of options if you need them, and has an informative video on Youtube, and a good FAQ page (links on add-on page).

doesn't work on youtube, detects only some small files that can't be played

Wow! This video downloader addon is just TOPS!
Despite my computer's streaming, graphic, CPU, gaming and high speed internet capabilities which are all decent,
some streaming websites really suck and give me a hard time trying to stream movies! (the dreaded buffering circle of death, buffering every 5 seconds, films restarting halfway through, etc,.)
So I've had enough of their games! Time to download them whole and watch them later!
Using EVD (Its about time, thank you!) I just forget all about that bull-crap and download all of my favourite films right to my hard-drive, I choose when to watch them whenever I feel like it, whether I'm online or offline.
I encourage everybody to support the genius behind this marvellous addon!
Cheers!