Return is the same as enter. On PC's it's called enter and on macs it's called return.
I'm talking about this post, Barack Obama Ignites A Long Over Due Powder Keg , where between the first and second paragraphs there is no blank line. It is because the first paragraph is enclosed in div tags and div tags do not produce the blank line between paragraphs. All other paragraphs in that post have blank lines between them.
Take a look at this image, http://opposablethumbz.wordpress.com/files/2009/08/ed-format.png , which is of the top left of the editor window. When you get ready to start a post, and you put the cursor into the text area to start writing, make sure that in that bottom left pulldown it says "paragraph." If it does not, select paragraph from that pulldown before you start writing.
I use a mac exclusively with FF and Safari for writing posts, and have virtually no problems. If you do not get a blank line between paragraphs, then your editor has somehow lost that "paragraph" format setting. Why, I cannot say. If you ever hit return and do not get the blank line, highlight the previous paragraph and then select "paragraph" from that pulldown and then you will get the blank line.
For any previous posts where you do not have blank lines between paragraphs, open those posts in the editor, highlight everything in that post, and then select "paragraph" from the format pulldown and it should correct it for you.
Also if you have not, go to settings > writing and make sure that "WordPress should correct invalidly nested XHTML automatically" is selected and then click save changes.