/usr/lib/pkgconfig/mozilla-plugin.pc is in firefox-dev 45.0.2+build1-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | prefix=/usr
sdkdir=/usr/lib/firefox-devel
includedir=/usr/include/firefox
Name: Mozilla Plug-In API
Description: Firefox Plug-In API
Version: 45.0.2
Cflags: -I${includedir} -DXP_UNIX
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