It's not just replacing the Firefox brand, it's replacing the onus of responsibility for the package. When Debian packages your software, they insert their own

It's not just replacing the Firefox brand, it's replacing the onus of responsibility for the package. When Debian packages your software, they insert their own patches, they do their own disparate issue tracking and release archive management, they are quite effectively a separate product. If they called it Firefox, by Debian, it would be equally damaging to the Mozilla mark (in a sense of trademark law.) If they didn't do their usual Debian thing with the patches and the release management, it would be similarly damaging to the Debian mark, so-to-speak. (All packages get this treatment, no upstream software release vendor gets special privileges.)