Some nasty things can happen to your design if your files are saved with a Byte-Order Mark (BOM) character at the beginning. We had a view that was totally f*cked up in Firefox by this. The view was simple:

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Some layout around a
<%= yield %>
or around a
<%= @content_for_layout %>
and a secret character gets put in.. it's the BOM!

In our view (in Firefox) there was a new-line added and a div was moved and had a spacing. To remove it check here and use the script here!