I see what you mean. Your menu was more intricate than I thought first, and my 'patch' will not work. I remember IE treats inline elements incorrectly when it comes to paddings, borders, and margins, hence the different rendering than that in FF. Ironically, IE shows it the way you want!
The basic problem is how to 'center a float'. I don't know if this is possible using some technique. Maybe some expert out there knows.
Until someone comes up with a suggestion, try to rework your code using a table. I know that this is somewhat like reverting back to the old days, but you would get what you want. Sometimes it is necessary to revert back to tables because of lacking browser support.
Without the :hover and the :hover emulation script for IE (which you can add the same way) you could start out with this:
background: url("images/button_left4.gif") no-repeat left top;
background: url("images/button_right4.gif") no-repeat right top;
<table align="center" cellpadding="0" cellspacing="0">
<td><div class="slide"><img src="images/icons/building_add.gif" border="0">Add Property</div></td>
<td><div class="slide"><img src="images/icons/building_add.gif" border="0">Second Button</div></td>