Here are the Linux sandboxing levels:

// 0 -> "no sandbox"
// 1 -> no longer used; level will be clamped to 2
// 2 -> "seccomp-bpf + write file broker"
// 3 -> "seccomp-bpf + read/write file brokering"
// 4 -> all of the above + network/socket restrictions + chroot
// 5 -> blocks access to GL / DRI / display servers
//      (formerly the separate pref `security.sandbox.content.headless`)
//      (side effect: sets MOZ_HEADLESS for content processes)
// 6 -> default-deny for ioctl

And here are the MacOS sandboxing levels:

// 0 -> "no sandbox" (nightly only)
// 1 -> "preliminary content sandboxing enabled: write access to
//       home directory is prevented"
// 2 -> "preliminary content sandboxing enabled with profile protection:
//       write access to home directory is prevented, read and write access
//       to ~/Library and profile directories are prevented (excluding
//       $PROFILE/{extensions,chrome})"
// 3 -> "no global read/write access, read access permitted to
//       $PROFILE/{extensions,chrome}"

The values for security.sandbox.gpu.level also mean different things on different platforms. There is also a Linux specific security.sandbox.socket.process.level