boas, eu uso um que adiciona uma barra acima da página, e também tenho para o sapo vídeos que eu fiz (acho k anda por aqui no PaP)- funcionam perfeitamente no firefox.
o do youtube:
// ==UserScript==
// @name YouTube to me v2
// @namespace http://home.comcast.net/~teridon73
// @description Provides download links for Flash FLV files hosted by YouTube.com
// @include http://www.youtube.*/*
// @include http://youtube.*/*
// ==/UserScript==
// YouTube URL: http://www.youtube.com/watch?v=[video_id]
// YouTube download link: http://youtube.com/get_video?video_id=[video_id]&t=[t_id]
var download_url = 'http://youtube.com/get_video?video_id=';
var playerDiv = document.getElementById('movie_player');
var flashvars = document.evaluate("attribute::flashvars", playerDiv, null, XPathResult.STRING_TYPE, null).stringValue;
var t_id = flashvars.match(/t=([^(\&|$)]*)/)[1];
var video_id = flashvars.match(/video_id=([^(\&|$)]*)/)[1];
var video_url = download_url + video_id + '&t=' + t_id;
// add banner with download link
var my_banner = document.createElement("div");
my_banner.innerHTML = '<div style="border-bottom: 1px solid #CCCCCC; margin-bottom: 10px; font-size: small; background-color: #70859B; color: #FEFEFE;">' +
'<p style="margin:0px;padding: 5px;text-align:center;">' +
'<a href="' + video_url + '" style="color:#FFFFFF; font-weight:bold; font-size:10px;">"Save As" to download Flash video</a>' +
'</p></div>';
document.body.insertBefore(my_banner, document.body.firstChild);
// clean up body margin
document.body.style.margin = '0px';