Download it from this link: http://www.jjamwal.in/dl/nodofollow.xpi
Open the XPI file from Firefox’s Open menu and it’ll be installed.
In case, you want to know how it’s done, read on.
There isn’t much to it. XPI files are just ZIP files with modified extensions. All one needs to do is unzip the file into a folder and open install.rdf file in a text editor. Find and modify the following line em:MaxVersion=”3.6″
Replace 3.6 with any version number of Firefox you want. I changed it to 6.6. :p
After that, all you need to do is to create a zip up these files in to an archive again and change the extension to XPI from ZIP. Take care of not zipping the folder in which you extracted the files. The archive should be of content structure as it is.