Why can’t you expand the storage on your root server?
The firefox downloads should be going into the WebCache, which should roll over automatically as you run out of space, so it shouldn’t cause a problem.
If you cannot expand the storage on your root server, then you could have a single top level relay that you set to do the downloads directly itself rather than the root server. This would take the burden of web downloads and storage off of the root server and put it on the top level relay, which is much easier to swap out and rebuild as needed.
If you are going to stick with the plan as you have laid out, then it seems to me you are already going to be making custom Firefox tasks, so why not just make them for the languages you need and just remove the others? There is no reason to need dynamic downloads if you just remove the references to the downloads you know you will never need.