Firefox renders buttons and links in action list inconsistently. There is a 2px extra spacing around button element. You can see that on this screenshot:

"Subscribe" label is not in line with other labels. It happens, because "Subscirbe" is a <button>, whereas other menu elements are <a>.

This problem with Firefox is well known. Check this for example: http://stackoverflow.com/questions/5517744/remove-extra-button-spacing-padding-in-firefox

Adding this CSS rule solves the problem:

button::-moz-focus-inner {
    padding: 0;
    border: 0
}

Screenshot after: