Hi Connemmara
Click on the firefox logo to download it (7.8MB)
Go to the folder you downloaded it to and double click on the firefox application to install it - just follow the instructions it gives you.
Go to the addons page and click add to firefox.
now when you watch a utube vid there will be an animated icon next to the address bar with a small arrow on the right hand side, click on that and a drop down menu appears giving you your download options.