I've been very frustrated recently with the fact that in facebook groups, if you press enter while typing a post, it submits the post. If you just want to make a line break you have to hit shift-enter.
Fortunately we don't have to live that way any more. I've made a greasemonkey script for people who use firefox. You can either install it directly or install it from the UserScripts.org site
To be perfectly clear:
You must be running firefox http://www.getfirefox.com
You must have GreaseMonkey installed: https://addons.mozilla.org/en-US/firefox/addon/748/
BEFORE you can install my script.
After you have installed my script when you are on a group page, press the alt-z to turn on safe typing. Now you will be able to press the enter key all you want to make line breaks. Then when you are ready to save, you can press the alt-q to turn OFF safe typing and THEN press enter (sequentially, not simultaneously), and your post will save. You'll need to press the alt-z AGAIN to re-enable safe typing.