The Collusion add-on has not been published at the Firefox add-ons site. Instead, you can install it directly from the developer's web site. Go to this page and click on "install add-on":

https://secure.toolness.com/xpi/collusion.html

Hi mbrubeck,

Yes, I found that out yesterday night itself... Sorry I forgot to post the solution. However, firefox is stopping the install and asking for manual intervention of the addon - at which point I stopped.

I realised that what I am interested in really is blocking the trackers, and hence went ahead installing the addon 'TrackerBlocker' (which is a regular ffox addon - also reco'd in several Collections).

I will later tryout the collusion addon - probably on a Sandboxed FFox....

Thanks for the prompt help, -feltra