Darrell Norton posted this snippet for Firefox on his blog and it works like a champ. MSDN pages that list code used to look like this in Firefox:

After adding the snippet below to userContent.css, it looks like this:

Here's the snippet to add to userContent.css. As Darrell mentions, using ChromeEdit is by far the best way to customize the Firefox customization files. Even finding them can be a trick at first...

/* Microsoft MSDN code stylesheet */
pre
{
   white-space: pre !important;
}

Sweet tip... thanks, Darrell!