It's an add-on, but are you saying that you wish to grab the source because you can't get it working with firefox 29 or greater? That's the problem I had. I had to install "Classic Theme Restorer" in order to be able to allow me to see forecastfox. But otherwise, I thought that add-ons were precompiled XPI files and you gave them to the application which uses MIME to install them; so I didn't think there was Windows vs Linux vs other variations, but instead one install file which worked for any OS.
This is where I got
Classic Theme Restorer which you need for FireFox 29 or greater. And many thanks to
D IsMe May 7 Reply for that link.
And sorry I can't find a place to download the XPI file; also checking on my system to see if it kept the file, but no dice yet. I'll update if I do manage to locate it; I know what you're saying about it bringing you to a bunch of ad pages.