\documentclass{beamer}
\usetheme{Frankfurt}
\usecolortheme{dove}
\begin{document}
\section{Introduction}
\begin{frame}{Frame}
\end{frame}
\end{document}

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I did see it, but since I wanted to remove the shadows globally and from the header I didn't understand how to adapt solution to my question. –  Artturi Björk May 2 '13 at 22:09

Just add \setbeamertemplate{frametitle}[default][colsep=-4bp,rounded=false,shadow=false] to the preamble. See below.

\documentclass{beamer}
\usetheme{Frankfurt}
\usecolortheme{dove}