In the past, the hardest thing about learning CSS had little to do with the language itself, but in dealing with inconsistencies for specific browsers (like "Firefox", as mentioned).
Recent efforts by browser manufacturers to render CSS in accordance with standards, along with the advent of pre/post-processing tools, have made it easier for developers to focus on the language.
Fair, I've only been doing this a year, my older colleagues laugh strangely whenever I complain about browser inconsistencies. I'm of the understanding I live in a quite different world :P
