If you ever find yourself debugging some odd Firefox only issue, you might be able to find some clues in the default stylesheets of the browser.
If you navigate to resource://gre-resources/ in Firefox 4 you’ll find a list of various resources used by the browser.
As far as I can tell, the following stylesheets are the ones used to provide the base styles for all pages:
The styles in those files are what CSS reset stylesheets are trying to eliminate.
If anything, it’s interesting digging through those files just to see what kinds of tricks they use internally. There are lots of -moz vendor prefixes in there.
Thanks to James Oppenheim whose instructions for Firefox 3 lead me to the files in Firefox 4.