I have been using it for years, NO PROBLEM Rated 4 out of 5 stars
I really don't understand 1 star ratings complaining about having to pay for using certain features like converting videos. I never needed such feature(s), and I am a HEAVY user who installed this years ago. Obviously, I'd be happier if everything was free, although -seriously- you won't miss any features here without the need to pay for extra ones.
Besides, there are other nice little programs out there which will convert all type of files without a problem.
Apart from that, this add-on was featured 7th in the list of the slowest add-ons for firefox, apparently slowing it down up to 33%. I reckon Firefox have been crashing a lot in their latest versions, particularly when queuing more than 10 videos (so I suspect this add-on is involved in the crashes). However, I recently changed to WaterFox and it works fine. I never tried queuing more than around 20 videos yet (the time when I could queue more than 100 without a problem seem long gone), but so far so good.
I am still looking for a different light weight add-on capable of doing what this one does, Unfortunately, none so far seems to match it. So, even though DownloadHelper is a heavy weight with its more than 700kb, I'll stick to it for now and I thank the developers for it.