In all the browsers I’ve tested, my button looks like this:

Except in Firefox where it looks like this:

I’ve posted the code as a jsfiddle, what am I doing wrong?
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On a normal site, you should always include a reset to get rid of these small inconsistencies between browsers. Here’s a simple one: http://meyerweb.com/eric/tools/css/reset/
(Edit: Ok, you use normalize, that could also work)
However, in your case I would also suggest instead of using padding on the top and bottom, remove them and your height and instead use line-height! This will keep your text nicely centered everywhere.