I've used savevid but i think you loose some quality with that.

Try Video DownloadHelper. I believe it's the most popular one for FF:

Code:

https://addons.mozilla.org/en-US/firefox/addon/3006

I use Orbit Downloader which has the advantage of being able to download many forms of media, but I like the Mozilla add on as well.
Try whatever suits your computer best I suppose.

You can use keepvid.com to download or use IDM also..I use anyone from these two tools.. And my downloading has never stopped in middle.