As I mentioned at the end of this comment, the apparmor profile torbrowser.Browser.firefox prevents Tor Browser version 10.0 from loading libstdc++.so.6. It results in the following error on the command line

./firefox.real: error while loading shared libraries: libstdc++.so.6: failed to map segment from shared object

and the following message in the audit logs:

AVC apparmor="DENIED" operation="file_mmap" profile="torbrowser_firefox" name="/home/<user>/.local/share/torbrowser/tbb/x86_64/tor-browser_en-US/Browser/TorBrowser/Tor/libstdc++/libstdc++.so.6" pid=3883 comm="firefox.real" requested_mask="m" denied_mask="m" fsuid=1000 ouid=1000

I tested that this error occurs on both version 0.3.2-13~bpo10+1 in Debian buster-backports and from building from source on develop/HEAD.

After looking at this further, I found that the reason this happens with Tor Browser version 10.0 and not the previous versions is because the system-wide libstdc++ in Debian Buster is too old for this version, so these lines in the shell script firefox (distributed with Tor Browser) decide to use the bundled libstdc++ instead of the system-wide one:

# Check if the system has a more recent version of libstdc++.so.6; if yes, use
# that instead of the bundled version.
"$basedir/abicheck" >/dev/null 2>&1
if [ $? -ne 0 ]; then
  add_LD_LIBRARY_PATH "$basedir/TorBrowser/Tor/libstdc++/"
fi
add_LD_LIBRARY_PATH "$basedir/TorBrowser/Tor/"
export LD_LIBRARY_PATH

To confirm this, running ./abicheck shows that the required versions are indeed not found:

./abicheck: /lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.28' not found (required by ./abicheck)
./abicheck: /lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.26' not found (required by ./abicheck)