You have to create it. Go to <code>about:support</code>. Look down the blue block on the left of the screen that will open until you come to Profile Directory. Click the Open Directory button next to it. This will open your Profile Directory in Windows Explorer. Make a directory/folder in that called chrome, all lower case. In that, ie the directory you've just created, put your <code>userContent.css</code> file. If you haven't already you need to go to <code>about:config</code> and set <code>toolkit.legacyUserProfileCustomizations.stylesheets</code> to <code>true</code> for your changes to work.