Your code doesn't match in the example you've posted here.
myulclass.hours doesn't exist in your HTML.
However, your issue is likely defaults between browsers, because that's what people commonly run into.
I can never keep it straight who does what: someone has default margins on ul's and someone else has default padding on ul's. This was a famous difference between IE and Firefox, though all browsers add something to make room for bullets.
First, always keep the stuff around a list simple: why float something and then in the markup say "display: inline"? You know that inline markup has ultimate say? Do you want content_right to be inline or a block?
You're not using list-item so it's really just a matter of the padding on the li's to make room for their background image.
So what you want to do is give everyone an ugly background colour. I won't bother listing them because the HTML and CSS didn't match up here (I assume they do in your real code), but just make it a different colour. Between browsers, you'll probably see someone adding padding or margin to the left side of the ul and the other browsers aren't, showing the reason for the discrepancy.
*edit another thing occurred to me. IE6 and 7 have something called Haslayout, which affects lists when the list is next to a floated something (an image for example). IE8 doesn't have this nasty Haslayout though, so if there's other pertinent markup around this list, it may have something to do with this.
You might want to post a live page, even if it's a dummy version.