ELI5: how is any of this legal? Let’s say my distro receives Firefox source code under the terms of MPL, builds it and distributes it to me under the same terms

ELI5: how is any of this legal? Let’s say my distro receives Firefox source code under the terms of MPL, builds it and distributes it to me under the same terms. At no point any of us agreed to any additional terms. Does this apply only to Mozilla-built binaries?