Ubuntu12.4如何设置firefox为系统默认浏览器

今天发现ubunto12.4没有默认的浏览器，导致所有的链接打开的时候从用gedit。google上找到了解决方法:
编辑以下内容：

[ Desktop Entry ]
Version = 14.0
Name = Mozilla Firefox Web Browser
Comment = Browse the World Wide Web
GenericName = Web Browser
Keywords = Internet ; WWW;Browser;Web;Explorer
Exec = /opt/firefox/firefox %u
Terminal = false
X-MultipleArgs = false
Type = Application
Icon = firefox
Categories = GNOME ; GTK;Network;WebBrowser;
MimeType = text/html ; text/xml;application/xhtml+xml;application/xml;application/rss+xml;application/rdf+xml;image/gif;image/jpeg;image/png;x-scheme-handler/http;x-scheme-handler/https;x-scheme-handler/ftp;x-scheme-handler/chrome;video/webm;application/x-xpinstall;
StartupNotify = true
Actions = NewWindow ;

到文件~/.local/share/applications/firefox.desktop, 并保存退出。
执行：

update-desktop-database ~/.local/share/applications/

配好以后"System Settings -> Detail -> Default Applications -> Web" list里就会有firefox了。

Ref: http://askubuntu.com/questions/166455/how-do-i-make-luakit-my-default-browser