It's pretty simple, really. We have specific permission from Mozilla to make some very specific modifications to Firefox and redistribute it as "Mozilla Firefox, Portable Edition" and keep the Firefox trademarks intact. Those permissions do not extend to 3rd parties (others can redistribute our unaltered binary installers, but can't repackage it). So, we can't make the modifications in question and then distribute them as a Firefox package. Nor can anyone else take our work and make those changes and still call it Firefox and keep the Firefox logo without similar permission/licensing from Mozilla. Trademark law allows them to protect their trademark in that fashion and prohibits anyone from distributing a product with their trademark on it without their permission.