When you click on Download@Authors Site you get to the next page and the download from the author's site begins. Period. If there is something amiss there then I have never, ever had it happen to me. And I download a fair amount of software from there. Trust me, I am VERY careful on the net. Learned my lesson a long tiome ago and I regularly work on infected computers for friends and family.
I have never had an issue at Major Geeks. Ever. Their download link for Firefox is as clean as a whistle. They do not ever redirect me to any place else. Ever. This is where I first saw that Beta 3 was available and the file that is downloaded from their link is the very same, exact file that is downloaded from the official Mozilla site that chachazz posted. Nothing added. No spyware, no nothing added. On all of their downloads they have a link directly to the software's host site. I daily check Major Geek's, Beta News, Download Crew, FileHippo, and Neowin.net. End of rant.