I am sorry to hear that you are having problems preparing posts from your Mac.
I am writing this post using the latest version of Firefox and in my view firefox works better than Internet Explorer with the new forum.
So, as you say we must assuming some kind of setting issue and I suggest as a first step I will have a look at the HTML associated with your post. Perhaps when you next post, after you have finished writing, click the HTML button which opens the windows to see the HTML code and paste it underneath your text you have written.