I'm comparing how it's handled by Chrome vs Firefox
In Chrome you go to your profile, check a box to confirm that you want separate shortcuts, done and it's handled properly when merging multiple windows opened by the same user. Each icon is visually distinctive as well.
In Firefox there's no native solution to have separate icons for each profile, the way to do it is to create a shortcut to the .exe file and to edit the path so the shortcut opens Firefox with a specific profile selected. Because the new shortcut isn't the "regular one", the windows don't merge under the existing profiled icon in the taskbar, they instead add a separate icon in the taskbar where the windows merge, it means that you end up with two icons to open Firefox (one for each profile) and two icons where you actually find the windows currently opened. Add to that the fact that because it's just "regular shortcuts" under the hood, it ignores the custom icon you're using to differentiate between profiles (again, because it's not a native solution) when creating the new icon where the windows are merged. You end up with two profile icons and two default icons and the only way to know which one is yours is to go over it to see what windows are opened underneath. Three users with each one having their own profile? That's six icons in the taskbar if everyone has windows opened, three of them with the actual instances "in them", all three using the same icon and they're not in a specific order.
The (native) alternative AND official way to handle profiles in Firefox? Open about:profile every time you realize you're browsing under the wrong profile.
There's no real user-friendly solution. Downloading an extension to fix a UX issue is ridiculous, that's on the actual devs to make it native. Installing Firefox twice (one beta and one regular) is a waste of space and potentially exposes one of the two users to vulnerabilities from using a pre-release versions of the program.
When I mention that issue the reaction is always the same as yours "Don't see the issue with it" from people who haven't compared to the alternative or whose use case has nothing to do with two (or more) person using the same computer and only needing separate browser profiles and having no reason to need separate OS profiles.