Estos son algunos complementos que me han resultado utiles.

• PDF Download: PDF Download relieves the pain experienced when encountering PDF files
  on the Web. Whenever you click on a PDF file, PDF Download lets you
  know before trying to open it, and then offers you choices such as
  downloading, opening, or converting it straight to HTML
• Video download helper: ayuda a bajar archivos de YouTube
• DownThemAll: adminstrador de descargas
• FireFTP: cliente de FTP

Enjoy it!