tonearm, I'm unable to duplicate the problem you've described, either. The code snippit doesn't give us a very complete picture of your page. For example, does it have a valid doctype, are there other styles involved, are you indeed using an XHTML doctype, etc? If you can post a complete working example that demonstrates the problem, we'll have a better chance of identifying the problem. Please click the link to our posting guidelines at the bottom of my message and post a complete page that demonstrates the problem... starts with <!doctype, ends with </html>. Help us help you.
I checked Bugzilla but only found one match for "box-shadow input" (without quotes) and a few reports when the search was for "box-shadow border-radius" but they didn't sound like this issue.
My very first post in the SitePoint forums was about a Firefox bug, so you might have found a new one.