What message editor interface are you using. I have never experienced any problems with the Standard Editor Interface on any of the browsers you mentioned. Remember this side uses BBcode and not HTML.
Do you have an example of a post? I checked a few of yours, and they look normal to me. For example this one.
I have the same problem. I even ended up adding all kinds of code to my post to get my sales thread look decent. Don't feel like doing this all the time. This is a new line, but how does it show?This is on the next line in the editor, how does this one show?What can be done?