I made the following css:

.mo {font-family: monospace; font-size: 9pt;}
td {border: solid thin #000000; padding: 4pt;}

With the following html (part of a table)

<td class="mo">abcdefg
 <br />hijklmn</td>

The text is not displaying in a monospaced font in Firefox. The i is narrower and the m is wider. The table cell width is not a constraining factor as the cell above it has wider content. The displayed font looks like Arial.

It displays correctly in IE.

Why

I found the trouble.

The font I originally selected in Firefox Options for displaying monospace has somehow disappeared from my computer. I selected a different font and it is now working.

I am now wondering what happened to that font.

I now know where that font went. It was included with Office 2003. When I replaced the hard disk, I installed Office 2010. I copied the Firefox settings from the old installation so I would not lose my bookmarks. But the font was not included with Office 2010.