css button active state causing text to move?

I'm just working on some rough examples for a new site i'm working on.

for some reason the later 2 of these buttons in firefox(latest) will move the text slightly as if implementing a click animation as they enter the active state. why would the first button not also be experiencing this issue?

code below

<fieldset style="width:320px; float:left;">
        <legend>Pink Button</legend>
            <button class="pinkbutton"><span>MESSAGE HERE</span></button>
            <button class="pinkbuttondisabled" disabled="disabled"><span>DISABLED</span></button>

    </fieldset>

    <fieldset style="width:320px; float:left;">
        <legend>Grey Button</legend>
            <button class="greybutton"><span>MESSAGE HERE</span></button>
            <button class="greybuttondisabled" disabled="disabled"><span>DISABLED</span></button>

    </fieldset>

    <fieldset style="width:320px; float:left;">
        <legend>White Button</legend>
            <button class="whitebutton"><span>MESSAGE HERE</span></button>
            <button class="whitebuttondisabled" disabled="disabled"><span>DISABLED</span></button>

    </fieldset>

css below

.pinkbutton { border:1px solid #D2247b; width:150px; height:30px; background-color:#EF0093; color:#FFF;}
.pinkbutton:hover {border:1px solid #FF4296; background-color:#FF5EAC;}
.pinkbutton:active {border:1px solid #A61D61; background-color:#DC2F85; color:#333232;}
.pinkbuttondisabled {border:1px solid #F3C4DB; width:150px; height:30px; background-color:#FDC6E2; color:#FFF;}


.greybutton { border:1px solid #BBBBBB; width:150px; height:30px; background-color:#E2E2E2; color:#8d8c8c;}
.greybutton:hover {border:1px solid #EF0093; background-color:#E2E2E2; color:#f62c92;}