Your statement is inaccurate. What would be accurate is something like “I couldn’t start Firefox without specifying the actual executable I desired.” Windows keeps a record of what executable to use when you specify an alias. In your case, specifying just “firefox” from the command line.
If you specify the actual executable, using normal methods of specifying an exact file, then this is not an issue. What you desired was to be able to use an alias/shortcut from the command line instead of specifying the actual executable and being sure you’re executing it from within the appropriate directory.
While it’s reasonable/beneficial to set up aliases/shortcuts, and other people probably will desire to do so, it’s inaccurate to state that you “need” to modify the Registry in order to execute more than one Firefox browser from the command line. What you need to do is accurately specify the executable which you desire to run (which is necessary on any computer, everywhere, for anything). There are lots of ways that exist that make it easier to specify the exact executable. What you’ve shown is one way to do so.
Part of what you experienced was probably Firefox setting the “firefox” alias/shortcut during the installation process. This would result in the alias/shortcut pointing to the version which was most recently installed. Thus, a corollary is that you should install last the version you wish to be the default, even if that means you reinstall a currently installed version.