So for the past few months I've been plagued with an issue in Firefox 2 that causes SolPartMenu and DNNMenu to add too much spacing to the navigation when a site is visited on first load.

Example:

The peculiar thing is that when you either refresh or go to a different page on the site, the issue is fixed.

Example after hitting refresh:

Well the other day I noticed that in Firefox I was getting a broken image link to each side of every nav item when the page first loaded but it only lasted a fraction of a second until it disappeared. A little patience and multiple cleared caches and I was able to right-click on the broken image link and found out that it was the spacer image. The curious thing is that the spacer image was there. So here's what I did to solve it:

PRELOAD YOUR SPACER IMAGE

I just used this small bit of
-----------------------------------
<SCRIPT LANGUAGE="JavaScript">

spacer = new Image();
spacer.src = "/images/spacer.gif";

</script>
-----------------------------------
Note: This is assuming you have a spacer image at the root level of your install which I believe is standard for all installs.

It looks like Firefox was too slow to load the spacer image and while even though it did eventually load, it still retained some of the space made by the broken image icon.