I highly doubt the fingerprint would be much different with the hardening enabled, if at all. The sandboxing shouldn’t be visible to websites as it doesn’t affect anything internally. hardened_malloc might change the fingerprint (you can see differences in about:memory) but I doubt websites can detect this.
The warning with the sandboxed-tor-browser project is likely because that sandboxing was part of the actual browser and not some external thing like firejail.
I also think we should be using something like bubblewrap for sandboxing for reduced attack surface but that’s a whole other discussion.