Try to fine tune button size.
IE has long button bug, to work it out try this solution.
FF also has a problem, which can not make it smaller (can't remove padding).
According to http://www.designdetector.com/demos/buttons-padding-demo.html,
3px padding left and right, and 1px padding top and bottom appears to be added even when zero padding is applied. Looking at bugzilla, I found Bug 140562 – button control with 0px padding still displays a frame around its contents. Hope it will be fixed in the 3.1 release.