Box Shadow in Firefox

I have a problem with box-shadow in Firefox - it "lags":

This is my CSS:

-moz-box-shadow : 0 0 5px #333;
-webkit-box-shadow : 0 0 5px #333;
box-shadow : 0 0 5px #333;

In Chrome, it works normally (without "lag"), but in Firefox it's slow.

How can I fix this?