There was a bug in Firefox 152.0.2. On my (non-Fedora) system the result was that if you closed Firefox then reopened it, the new instance of Firefox froze and rapidly swallowed up memory.
So it looks that version has been withdrawn from Flathub. When I did flatpak update from the command line I got the same downgrade.
You’re right, today a received the 152.0.3 update and everything is ok with both Firefox and Discovery.
To me at least 152.0.2 was fine so this is why I didn’t immediately see this a “fix” and instead viewed this as a problem.
I think Flatpak/Discover should raise a warning about the reason of the downgrading because I’m sure this same question or a related one was made on others forum/places