Opera 9 and IE6+ gets the image centered properly. So there's two answers:
1. Firefox gets it right as you can center it with line-height and other browsers get it wrong. 2. Firefox gets it wrong and it needs to be updated in the next version.
I would use 50% on the background, only my bullet lists have multiple lines, which puts the bullet below the top line.
Does anyone know how to vertically center the list-item? My bullet is going to the bottom of the line instead of centering vertically - it's a small square so it looks like this: . line item text
If you want to center your background image on the first line of the li's text, position the background with em's rather than percent. As in
li { padding-top: 5px; background: url(bullet.gif) left 0.5em no-repeat; }
This way the bullet height will scale with the text if needed (move down if text expands). A little experimentation might be needed to get just the right positioning. (Make sure to view it with text enlarged to get acceptable behavior in that scenario.)