After bashing my head against a wall for a while trying to figure out why one page in a site was dropping my floated sidebar div to the bottom of the page, I came across this:
http://www.davidbisset.com/2007/12/20/drop-down-list-breaks-float-layout-in-firefox/
Say you have two divs – both have floats so that they can end up being two columns on your site. But sometimes when you add a dropdown menu (<select> tag) to one div) it will break the layout… usually meaning that the other div will be pushed down. And this usually happens only in Firefox. IE sees it just fine.
The solution to this? Instead of the nice linear coding that you’re used to where the sidebar, to the right of the main area, comes after the main area code, it needs to be before the main area code.
Because apparently, in FireFox 2 seems to have a problem with floated divs, option tags, and SIMPLE COMMON SENSE.