http://www.w3schools.com/css/css_syntax.asp
What I'd recommend is starting completely over. Just trash your existing userChrome.css file. Make a brand new one with absolutely no content other than these exact lines:
Code: Select all
/* Attachment pane at bottom of message window */
#attachmentView {
-moz-appearance: none !important;
height: 40px !important;
overflow: auto !important; }
Just copy and paste these lines into your empty userChrome.css file, and then save it. (And of course, make sure it's in the "chrome" folder in your profile folder.)
If that works when you restart TB, then you can exit TB and add some other CSS to your userChrome.css. Just add one item at a time (such as font size), and test it before adding anything else. Take it one step at a time, and it should work.