/usr/bin/firefox is in firefox-esr 52.8.1esr-1~deb8u1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | #!/bin/sh
FIREFOX="$(which firefox)"
[ -x "$FIREFOX.real" ] && exec "$FIREFOX.real" "$@"
exec firefox-esr "$@"
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