Your Ubuntu system now is running on the new windowing system Wayland instead of the traditional system Xorg. This is fundamentally different, and many of the "good old" tools that interact with Xorg do not work on Wayland.
Some applications that do not support Wayland run on Xorg through an interface laye, xwayland. These tools will continue to work for these applications.
The Snap version of Firefox actually runs on xwayland for me. So for the default version, the tools would work. Firefox installed from third party sources via the APT system natively run on Wayland, so the tools do not work.
You can tell whether an application runs on xwayland if it appears in the output of wmctrl -l in a Wayland session.
So here is why you cannot use xprop.
If these tools are important in your workflow, you still can run Ubuntu on Xorg instead as of old (I personally do). You will loose touchpad functionality, and in particular one-to-one mouse gestures. Wayland supports different scaling factors when using different monitors, Xorg does not. In some cases, Wayland animations may be noticeably more smooth than on Xorg. In the end, you must make the balance and choose for yourself what you prefer to use.
