> So the distro will have to wait for a new release of the package before
> they could include it?
Exactly. Ideally, distros shouldn't be shipping the software at all. If the vendor packages it, and you have the vendor's repository in your software update list, then you can update that package (or not) independent of the distro's time line.
> How is an extra step *easier*? I can't think of any distro thats not
> extremely niche that doesn't include Firefox in their default
> repositories. Mozilla also has a tendency to not support older versions
> of Firefox that some distros ship.
In the following, let us assume that Firefox could be any application.
What is easier is that one can upgrade their operating system or the application independent of each other without having to learn the details of system administration. Right now, if I want a new version of Firefox, I have to upgrade my entire operating system which I do not want to do. This will also upgrade all of the other packages on my system. That's a big change just to get a new version of one program.
I could install Firefox from source, or install the binary from Mozilla, but then I am venturing into system administration territory. I have to open a shell and type commands, figure out where to put the files, adjust paths, etc.
Contrast this with Windows users who have the ease of just double-clicking the icon to start the installer. Windows users can upgrade software without having to upgrade everything else on their computer. Likewise, they can upgrade their OS and then reinstall older versions of application software with the same ease as installing the new.
Modern linux distros assume that everyone wants to run the latest versions of software on their systems. For people such as myself who may want to run a mix of software versions there is no easy way to do so. No current Linux distros cater to my desktop needs.