Thanks, Peter and Ed.

Peter, I think you're misunderstanding: with all the YT downloaders I can find, Open Candy, which is widely seen as malware, is packaged with and integral to the downloader, so that's my issue. I have no issue with Firefox in terms of malware, just in terms of the quality of the film once downloaded, as the picture is much degraded in comparison to the original, even on the highest setting.

Ed, I'll have a look at keepvid.com and clipconverter.cc, thank you.