For some of my users, every post looks as if it's been quoted, with lines above and below each post. This happens very often on Safari, Chrome and older versions of Firefox.

http://img213.imageshack.us/i/screenshot20091027at206.png/

that's what it looks like on Safari

http://img35.imageshack.us/i/screenshot20091027at209.png/

this is what it looks like on current Firefox

what can I do to fix this situation?

I guess I am missing it completely.
I looked at both images and I really don't see much difference.
It looks like it is placing a horizontal rule above and below the post in the summary page.
Can you pinpoint the issue with more detail?
BTW,
You will never get it to display exactly the same across multiple browsers and versions.
You just may need to target one or two browsers and deal with the style settings to address the items

Thanks Ruben

Basically when you look at a thread, every reply has a white horizontal line above and below each post, in addition to the red line that we desire. I don't mind color differences or slight font issues. The problem is those white lines above and below each post that are visible in the first screen shot and do not appear in the second.

Try clicking the "full size" links on the screenshot pages, it's easier to see it that way.

Or if you have multiple browsers you can check out our forum at

www.redlettertribe.com

I have ie8 and firefox3.5 and I don't see any horizontal rules at all. I know you stated Safari and chrome and older Firefox versions.
But that is my point with cross browser compatibility.

What browser are you using to see the lines?
What browser do you wish for the lines to work correctly on?

Viewing http://message.stateradio.com/cgi-bin/ubb_cgi/ultimatebb.cgi?ubb=get_topic;f=2;t=003579;p=1

I see items missing using the browsers I use listed above.

Such as the blue arrow for new posts in the left top area shows for Firefox but not for IE8.
Then to the top right where you can navigate with the arrows for older newer topics is missing the explanation text to the left of the arrows on both browsers.

You might try a quick fix for IE8
By adding in your html header includes;
By adding the meta tag:

<meta http-equiv=”X-UA-Compatible” content=”IE=EmulateIE7"/>
It may work for IE8.