1) there are some good youtube download addone for firefox so that if your desire is to record youtube you might try one of those.
2) as for filter keywords -- these are only used to REDUCE the # of urls shown, so leave that blank always, unless you get too many urls shown.
3) im not understanding your think abour commandline $url stuff.
your comment here troubles me:
toggled off "only this computer" (packets stop if I turn this on)
are you on a network? i wonder if somehow its scanning packets from your network other computers and not yours.
how about try checking that option even if it looks like packets stop, but then click sniff and go to
www.c-span.org, and hold ctrl+shift while you hit the refresh button in your browser, or just go visit some other random sites, and see if the packet counter at the bottom statusbar ever increases.
try some sites you've never been to before like:
http://www.di.fm/