Hey I used this method too but it works on youtube for me, I have tried it on xvideos.com and some other sites but I get nothing
I've saved many from xvideos.com, and I just tried it again. No problemo. xvideo files store in flv format. But you have to wait until the whole viddy finishes before stored file shows up.
There are some sites that play the clips in formats that aren't stored in your temp files. Why they go through the trouble, who knows (guess they're selling something) but there are enough other sites, so to hell with 'em.
And some other sites store them in strange formats that don't appear to be video files. I use a file converter (Super C by eright soft) to manually convert them or just to view 'em. Sounds like the other referenced "Download Helper" does the same but more automatically (as an adjunct to Firefox). I prefer manually moving them from temp files to wherever I want 'em and converting those I chose.
Oh, btw, I found the storage cache for firefox. On my pc it's located at:
c:\documents and settings\(
your name)\local settings\application data\ Mozilla\Firefox\Profiles\(
an alphanumerical string).default\cache