I know that Firefox is still in beta, but maybe this is an issue that can be resolved easily. It seems that there is a difference in alignment of the Milonic menu in Firefox and Internet Explorer. 2 Pixels seem to be the difference. See the images below for the problem.
We are going to need a URL to be able to confirm this.
I spoke to one of the Mozilla team the other day and he said that the Mozilla 1.8 code will be used in Firefox 1.0. So, as Mozilla are currently at 1.8 Alpha stage, the release of Firefox is not too far off, now would be a good time to get these bugs looked at I suppose.
A URL would be really helpful as Andy says. Also, check your css. One thing I have noticed when there's a left margin issue a lot of times it's the difference in how the browsers have their internal, default margins set for the page, or sometimes it's how the css has different margins or padding coded. If that's the issue, you can test your different css margins and see if any are doing that, you can set your {body margins to 0 then all browsers should be starting at the same point, no margins. Some people set that in the body tag using these two codes topMargin="0" leftMargin="0" and MARGINHEIGHT="0" MARGINWIDTH="0" which works but throws non validation errors.