In a rather bizarre twist of fate, Firefox Mac is no longer holding paragraph breaks (blank lines between paragraphs) but now Safari 3 is. If you use the
<br /> between paragraphs (by switching to the code tab), it is stripped out when you click save, save and continue editing, or publish.
I just did three test posts in Safari 3 and paragraph breaks are now holding when using the visual editor.
Mac Firefox 126.96.36.199 users if you are having this problem, try posting in Safari 3. It is no longer available from Apple as a separate download, but is included in the 10.4.11 upgrade. If you are using Safari 2.x, give it a try and see if it works. Since I no longer have Safari 2.x I cannot test it.