Well, to judge your first CSS, I would say its really a good piece! I like it!
But there are certain things you should look after, sorry If I'm a bit picky! Actually I'm very perfectionist by nature...
1)As you already said, the comment background needs some fix (it's position is different in firefox and IE)
2) The design that you've put above and below the journal looks huge to me. So, if you see the journal in a lower resolution the images gets cut out from the right.
3)Yes, the sidebar needs a fix! As again, in lower resolution it overlaps with the main body text!
4)I think you should remove the hyperlink underline from the sidebar links! i.e. use no text decoration
5)Coding the mood list was good to do but I guess in this design it wasn't necessary, you probably could have put the mood list in the lower red part of the journal design with transparent backgrond and different font color
6) Try putting some padding in the thumbs and avatars. Or may be you can even put some borders along with hover color change.
Now don't get down-heart by seeing my the comments above because..
1) You've chosen smart font colors
2) The finish of your journal design is perfect
If you still need some assistance you can ask me.