They can't use the names FireFox, Mozilla or Thunderbird because they're trademarks of the Mozilla Foundation and the Mozilla Foundation requires that you either ship their software in pristine upstream state with no extra patches or strike a special deal with them in order to use their trademarks. Neither of those are acceptable to Debian because the first means that the software won't be properly integrated into Debian and that they'd have to ship new versions in stable because Mozilla never backports security fixes and the second is unacceptable because the rights that Debian would receive from Mozilla aren't transferable to their users.