I've made another firefox extension to make it easier to extract the mp4 video from the flv files.
https://addons.mozilla.org/en-US/fir...lvuncontainer/
It basically is just a front end for the "ffmpeg -i filename -vcodec copy -acodec copy newfilename" command.
Lemme know how it goes. I don't have a windows machine atm, so if anyone could confirm that it works ok on windows, that'd be great.
Note: it only works on Firefox 4.