Yes, the +1 was in response to rahber's comment. Digging through multiple CSS files is a pain.
If Firebug won't work, CSS Tidy might do the trick:
http://csstidy.sourceforge.net/
I also wrote a CSS parser class ("CSS magic") for WP-United that parses CSS, performs operations (modifies selector specificity -- you don't need this) and then spits out the result:
http://www.wp-united.com/svn/0.8.5/root ... -magic.php . However, it doesn't combine duplicate selectors (on purpose; CSS order can be important). It would be very easy to modify the addSelector() function to combine duplicates, but you'd also want to split selectors by ',' earlier than CSS Magic does it. You're welcome to use it... (gpl, usage examples at the top), but I expect a pre-built tool like CSS Tidy might work without any modification.