Generally, using an app from a deb package is preferred, I agree. But for Firefox, I find the limited sandboxing that Flatpak offers to provide some value.

Another advantage that a deb package will have is that they tend to be smaller than a flatpak that needs to package all of its dependencies – which is where the mozillateam Firefox build does not excel: at 232MB (expanded) vs. 239MB of the flatpak, it is not a big difference (I’m not accounting for the org.freedesktop.Platform runtime it relies on, as I use that with other flatpaks that I don’t have debs for).

Lastly, for some reason the mozillateam Firefox build always crashes for me when sharing screen from Wayland – I’m not sure why, but hunting that down is a bridge too far for me.