I think it is “safe” as in secure. But not “safe” as in private. QubesOS is mostly a “security” oriented OS. It CAN BE A PRETTY GOOD privacy oriented OS, as well, if you use Qubes-Whonix inside, in which case, you will be using Tor Browser, anyways.
All in all, the thing mozilla is doing is a shameful display, but shouldn’t be a problem for qubesOS users. We have our compartmentalization, we have our disposables. We have our Qubes-Whonix. We can install Mullvad Browser. We should be OK.