NoScript, tell it to allow scripting, then try it again.
click on the agreement, you'll have to tell it to allow downloads from http://www.siteadvisor.com/
After you do this once or twice, it will download and install, then restart Mozilla Firefox.
I took a lot of the utilities off my laptop cos of problems. Now, when I try to install SiteAdvisor it either takes me back to the pre-install screen, o just gets to 'the end' of the download and goes off. Either way I don't have site advisor - anyone know how to fix this?