userContent.css in fresh install of PortableFirefox 1.5

If i was using "userContent.css" where would that file go? Or what folders would i have to make...
From what i know of the profiles in Mozilla Firefox, there was a string of numbers folder then the profile folder where you can put the userContent.css file in the chrome folder (not sure how this helps...)

but i would like to know for a fresh install of PortableFirefox 1.5. Because i believe after a upgrade to 1.5.1 there are several new folders made that lead to the correct folder the userContent.css needs to be in to work.