The userChrome.css userContent.css do not exist by default, you have to create them, and that has always been so. I think Version 6 if not 4, and 5 before it do not create the chrome directory and you would have to create that first. If you updated to 6.0 you should have what you had before if using the same profile.

Changes to your userChrome.css require restarting Firefox, you might want to consider using the "Stylish" extension and styles so that you can test them immediately, even has a preview test mode. Stylish is now a very large extension and may be overkill for you though -- it does not use userChrome though as it controls it's own files.

You can access the Firefox Profile Folder via:

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder

See also:

• http://kb.mozillazine.org/Editing_configuration

You can use the ChromEdit Plus or Stylish extension to have easier access to the customization files.

• ChromEdit Plus: http://webdesigns.ms11.net/chromeditp.html
• Stylish: https://addons.mozilla.org/firefox/addon/stylish/