I have noticed the issue when I was using the User Agent Switcher add-on. This issue can also be reproduced by setting the general.useragent.override pref manually. Tested on Aurora 22 for Mac.
WFM in latest aurora Mozilla/5.0 (X11; Linux x86_64; rv:22.0) Gecko/20130513 Firefox/22.0 and nightly Mozilla/5.0 (X11; Linux x86_64; rv:23.0) Gecko/20130513 Firefox/23.0 Have you tested in a new firefox profile?
Yes, it can be reproduced with a clear profile. While stylesheets are listed on the left pane, the right page goes blank when I set general.useragent.override.
Thanks for the bug report - this is known.