Great! Firefox is also the app on my computer that does by-far the best job sandboxing untrusted code. It's good for us to be running most of our untrusted code in that environment, and even better for us to be blocking untrusted operations outside of that environment.
I genuinely don't understand the issue, it's a big security win if I can close off permissions on my system and have them open just for the apps that have good sandboxing around their untrusted code. Bonus points if the app in question allows per-domain screen recording permissions, which Firefox does.
I would go so far as to say that (one of) the reasons you run 99+% of your untrusted application code in Firefox is probably because Firefox is one of the very few places on your computer where it's safe to run that code. So it's a big win if we can start pulling some of those same sandboxing ideas into the native world; maybe that makes it easier to use more native apps for some tasks. It's a big win if I can treat apps on my computer the same way that I treat a random website.
Also keep in mind that "untrusted" is not a binary state. I pretty much only run apps on my computer that I "trust", but do they need to be able to record my screen? No, and I love the possibility of not needing to worry quite as much about what happens if they get compromised.
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There's a somewhat strange notion here that because people do everything in the browser per-app privileges don't matter. But in fact, the reason the browser's security model works and the desktop's doesn't is because the browser has per-domain privileges and permissions, and it's not that per-app permissions don't matter, it's that they do matter and the lack of them is why we centralize untrusted code into one of the few apps on our computers that has per-"app" permissions. Browser permissions work because they are granular, you can grant Zoom access to record your desktop without granting access to Facebook.
So maybe if the desktop copies that, maybe far in the future you won't need to run everything in your web browser just to feel secure. But as it stands with X11 I feel a lot more comfortable using a browser to record my screen specifically because it has privileges and permissions around that are segmented per-domain/"app". The lack of sensible security controls on the desktop is a contributing factor to why people don't use the desktop, and being able to lock that in and restrict the scope of where screen recording can happen does make me feel a lot more secure about my desktop.