I've been following Firefox Contributors’ Quick Reference and bumped into an issue at the ./mach bootstrap step.

It seems like ./mach build still works.

 0:05.44 Setting up artifact node.tar.zst
 0:05.44 Using artifact from local cache: /Users/mathjazz/.mozbuild/toolchains/508912413de80d5e-node.tar.zst
 0:05.54 rm tree: /Users/mathjazz/.mozbuild/node
 0:06.42 untarring "/Users/mathjazz/.mozbuild/node.tar.zst"
 0:03.99 Setting up artifact fix-stacks.tar.xz
 0:03.99 Using artifact from local cache: /Users/mathjazz/.mozbuild/toolchains/5d6ecaa378ef9f73-fix-stacks.tar.xz
 0:04.02 rm tree: /Users/mathjazz/.mozbuild/fix-stacks
 0:04.02 untarring "/Users/mathjazz/.mozbuild/fix-stacks.tar.xz"
 0:04.09 Setting up artifact minidump_stackwalk.tar.xz
 0:04.09 Using artifact from local cache: /Users/mathjazz/.mozbuild/toolchains/9c9afe77f1fcd488-minidump_stackwalk.tar.xz
 0:04.10 rm tree: /Users/mathjazz/.mozbuild/minidump_stackwalk
 0:04.10 untarring "/Users/mathjazz/.mozbuild/minidump_stackwalk.tar.xz"
 0:03.78 Setting up artifact clang.tar.zst
 0:03.78 Using artifact from local cache: /Users/mathjazz/.mozbuild/toolchains/a36e19412b7bfdbe-clang.tar.zst
 0:04.34 rm tree: /Users/mathjazz/.mozbuild/clang
 0:05.07 untarring "/Users/mathjazz/.mozbuild/clang.tar.zst"
 0:03.89 Setting up artifact cbindgen.tar.xz
 0:03.89 Using artifact from local cache: /Users/mathjazz/.mozbuild/toolchains/cd2e5937f8d6c45c-cbindgen.tar.xz
 0:03.91 rm tree: /Users/mathjazz/.mozbuild/cbindgen
 0:03.91 untarring "/Users/mathjazz/.mozbuild/cbindgen.tar.xz"
 0:04.23 Setting up artifact clang-tidy.tar.zst
 0:04.23 Using artifact from local cache: /Users/mathjazz/.mozbuild/toolchains/d7d8ddb2f750cbbf-clang-tidy.tar.zst
 0:04.46 rm tree: /Users/mathjazz/.mozbuild/clang-tools/clang-tidy
 0:05.19 untarring "/Users/mathjazz/.mozbuild/clang-tools/clang-tidy.tar.zst"
 0:04.61 Setting up artifact nasm.tar.bz2
 0:04.61 Using artifact from local cache: /Users/mathjazz/.mozbuild/toolchains/585ef8fa78b6e1e8-nasm.tar.bz2
 0:04.67 rm tree: /Users/mathjazz/.mozbuild/nasm
 0:04.67 untarring "/Users/mathjazz/.mozbuild/nasm.tar.bz2"
 0:04.23 Setting up artifact sccache.tar.xz
 0:04.23 Using artifact from local cache: /Users/mathjazz/.mozbuild/toolchains/1fe17c3d08977a91-sccache.tar.xz
 0:04.25 rm tree: /Users/mathjazz/.mozbuild/sccache
 0:04.26 untarring "/Users/mathjazz/.mozbuild/sccache.tar.xz"
 0:03.61 Setting up artifact rustc-dist-toolchain.tar.xz
 0:03.61 Using artifact from local cache: /Users/mathjazz/.mozbuild/toolchains/c2f73ff5728e2934-rustc-dist-toolchain.tar.xz
 0:04.34 Setting up artifact clang-dist-toolchain.tar.xz
 0:04.34 Using artifact from local cache: /Users/mathjazz/.mozbuild/toolchains/9a6f11f6e96f603e-clang-dist-toolchain.tar.xz
 0:02.83 Could not find a toolchain build named `macosx64-dump-syms`
Error running mach:

    ['bootstrap']

The error occurred in code that was called by the mach command. This is either
a bug in the called code itself or in the way that mach is calling it.
You can invoke |./mach busted| to check if this issue is already on file. If it
isn't, please use |./mach busted file bootstrap| to report it. If |./mach busted| is
misbehaving, you can also inspect the dependencies of bug 1543241.

If filing a bug, please include the full output of mach, including this error
message.

The details of the failure are as follows:

subprocess.CalledProcessError: Command '['/Users/mathjazz/.mozbuild/_virtualenvs/mach/bin/python', '/Users/mathjazz/src/mozilla-central/mach', 'artifact', 'toolchain', '--bootstrap', '--from-build', 'macosx64-dump-syms']' returned non-zero exit status 1.

  File "/Users/mathjazz/src/mozilla-central/python/mozboot/mozboot/mach_commands.py", line 55, in bootstrap
    bootstrapper.bootstrap(self.settings)
  File "/Users/mathjazz/src/mozilla-central/python/mozboot/mozboot/bootstrap.py", line 389, in bootstrap
    self.maybe_install_private_packages_or_exit(state_dir, checkout_root)
  File "/Users/mathjazz/src/mozilla-central/python/mozboot/mozboot/bootstrap.py", line 268, in maybe_install_private_packages_or_exit
    self.instance.ensure_dump_syms_packages(state_dir, checkout_root)
  File "/Users/mathjazz/src/mozilla-central/python/mozboot/mozboot/osx.py", line 343, in ensure_dump_syms_packages
    self.install_toolchain_artifact(
  File "/Users/mathjazz/src/mozilla-central/python/mozboot/mozboot/base.py", line 417, in install_toolchain_artifact
    subprocess.check_call(cmd, cwd=state_dir)
  File "/usr/local/Cellar/python@3.9/3.9.5/Frameworks/Python.framework/Versions/3.9/lib/python3.9/subprocess.py", line 373, in check_call
    raise CalledProcessError(retcode, cmd)