Dear DNN-guys

First, I've examinated Version 1.6.1.0 of the solpartmenu.
(If I do it not good, give me a answer...)

Let us have a look at the rootmenu.
A RootMenuItem is a table with one row and two colums (only things that give the layout...)

Horizontal menu
<td>
<table width="100%" CELLPADDING="0" CELLSPACING="0" border="0">
    <tr class="spmbar spmitm MenuBar cssMenuItem MenuItemCSS savecss="MenuItemCSS" saveselcss="MenuItemSelCSS" style="MenuItemStyle">
        <td NOWRAP="NOWRAP" TITLE="">MenuItemImage MenuItemLeftHTML-OR-&nbsp; MenuItemName MenuItemRightHTML</td>
        <td align="right" class="spmrarw cssMenuRootArrow">RootArrowImage-AND-&nbsp;</td>
       </tr>
</table>
</td>

Vertical menu
<tr>
    <td>
    <table width="100%" CELLPADDING="0" CELLSPACING="0" border="0">
        <tr class="spmbar spmitm MenuBar cssMenuItem MenuItemCSS savecss="MenuItemCSS" saveselcss="MenuItemSelCSS" style="MenuItemStyle">
            <td NOWRAP="NOWRAP" TITLE="">MenuItemImage MenuItemLeftHTML-OR-&nbsp; MenuItemName MenuItemRightHTML</td>
            <td align="right" class="spmrarw cssMenuRootArrow">RootArrowImage-AND-&nbsp;</td>
        </tr>
    </table>
    </td>
</tr>

What do we see?...

1 ::
As you will set a special class in the td of this table than you must use:
    classname td {}
or
    classname td img{}
or
    classname img.

2 ::
Solpart have a MenuItemImage then solpart return
    <img src="MenuItemImage.gif">
Solpart have not a MenuItemImage then solpart return
    'spacer.gif'

3 ::
Solpart have a leftHtml then solpart return
    LeftHtml
Solpart have not a leftHtml then solpart return
    '&nbsp;'

4 ::
The name of the MenuItem
I've not yet found it, but it seems that there is always a return before the MenuItemName and a return after the MenuItemName.
That means that there is always a gap within the td-tag. And this renders different in IE and in FF.

5 ::
Solpart have a rightHtml then solpart return
    RightHtml
Solpart have not a rightHtml then solpart return
  '' [nothing]

6 ::
Solpart have a RootMenuArrowImage then solpart return
    RootArrowImage and a '&nbsp' after it
Solpart have not a RootArrowImage then solpart return
    '&nbsp;'

Here you have an example of a selected RootmenuItem as it was rendered in FF (extraced with FireFox View Formated Source plugin) (only things that give the layout).
<td>
<table width="100%" cellspacing="0" cellpadding="0" border="0">
   <tbody>
   <tr saveselcss="RtMnItHo" savecss="RtMnItAc" class="spmbar spmitm MnBar RtAndSuMnItNo RtMnItAc">
      <td nowrap="NOWRAP" title="Design">
      <img src="spacer.gif">  [No MenuItemImage]
      <img class="RtMnItLeHtml" src="RtMnItLeHtml.gif" alt="rootmenuitemlefthtml">
      <img class="RtMnItBrArrw" border="0" src="RtMnItBrArrw.gif" alt="*">

      Design

      <img class="RtMnItRiHtml" src="RtMnItRiHtml.gif" alt="rootmenuitemrighthtml">
      </td>
      <td align="right" class="spmrarw RtMnArrw">
      <img alt="&gt;" src="ArrwDown.gif">[Here is for sure a &nbsp;]
      </td>
   </tr>
   </tbody>
</table>
</td>

When you see this then I'm sure that there is a gap (return) before the MenuItemName and a gap (return) after the MenuItemName. I think that IE and FF are rendering this not the same.
Look also at the RtMnItLeHtml and the RtMnItRiHtml
You can become that this way, if you use a image then you must put the class of the image in the token and also give a correct path:
<![CDATA[<img alt=&quot;rootmenuitemlefthtml&quot; class=&quot;RtMnItLeHtml&quot; src=&quot;/Portals/_default/Skins/StaSkin0101/RtMnItLeHtml.gif&quot;>]]>
<![CDATA[<img alt=&quot;rootmenuitemrighthtml&quot; class=&quot;RtMnItRiHtml&quot; src=&quot;/Portals/_default/Skins/StaSkin0101/RtMnItRiHtml.gif&quot;>]]>

Remark
You've also seen that NOWRAP="NOWRAP" is perhaps necessary for older browsers. I don't know... I've replaced it with style="white-space:nowrap;"

Perhaps... tomorrow I gone look at the menubreak (when I've the time)....
Hope this will help you...
  Is it so?...