Here's the latest version of my StumbleUpon Toolbar skin for Firefox. It includes the current version of the toolbar (2.4).
Note: You may have to set Firefox to allow installation from www.handheadman.com if you haven't done so before.

If you decide to go back to the original StumbleUpon skin, just reinstall it from here: StumbleUpon toolbar.

Oh, and if you're not using Firefox, you're missing out on way more than these few toolbar graphics. Make the switch, you won't regret it.