I don't understand why Debian has anything to do with Firefox. Why does the operating system have any say on what applications get released and how often? Isn't this the whole point of using free, open source software? To be able to choose your own components? Why do we need Debian bundling Firefox?
Because it's a curated experience. But debian (or any distro) don't force you to use their versions. You're quite able to install your own on your free, open source software. Find your own repositories, set up your own repositories, manually install downloaded packages, configure/make/make-install your own software... debian not only doesn't stop you from doing this, but is built around letting you do this (as are all major distros).
You're not forced to use a repository at all.