Bundled Themes: Twenty Nineteen dropcap appears higher than expected in Firefox.

Fixes the issue by adding moz specific top margin for the dropcap.

Props kjellr, joseph.dickson, maxpertici, samful.
Fixes #45876.
Built from https://develop.svn.wordpress.org/trunk@48047


git-svn-id: https://core.svn.wordpress.org/trunk@47814 1a063a9b-81f0-0310-95a4-ce76da25c4cd