ccache is no longer recommended for Firefox builds. Instead, use sccache.
Execute the following to add sccache to your mozconfig:
# NOTE: If your mozconfig is not named "mozconfig", update this command accordingly echo "ac_add_options --with-ccache=$HOME/.mozbuild/sccache/sccache" >> mozconfig
Make sure to run `mach bootstrap` to download a recent sccache to the correct location first. (Consult the sccache documentation for alternative installation or configuration instructions if you need them.)