I had the same symptoms, found the problem, and fixed it quite easily.
Problem:
In my case the problem was that I installed WP on a server with no domain name yet. So “wp-admin/index.php” was looking for “wp-admin.css” on the wrong server, ie. on mydomain.com rather than [my IP number] server where WP is installed. (I’m migrating servers and migrating from MovableType, if all goes well.)
Solution:
I uploaded “wp-admin.css” to where WP was looking for it:
https://mydomain.com/wordpress/wp-admin/
I just created a couple new folders to the server currently using the domain name and uploaded the CSS file. That sorted it.
When the domain name gets moved to the new server, it will work normally.
I’m using OS X 10.4.8 with Safari and Firefox. My problem had nothing to do with the browsers or the actual CSS code.