Which version of FF are they running?
"Firefox 3.6.28 and Firefox 12 are the last versions of Firefox that will work with the original version of Windows XP and Windows XP Service Pack 1. They won't be updated with security and stability fixes. To keep your computer and personal information safe, we recommend that you upgrade for free to Windows XP Service Pack 3 and install the latest Firefox. This article goes over your upgrade options."
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Maybe try Opera to see if its different? Maybe run your site through the older FF emulator and run firebug to see what needs to be changed in either the CSS or HTML files?
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Perhaps advice your users that they can get SP3 for free, and upgrade to the newest FF version? Which eliminates you having to hack your css and html files for the few that refuse to get into the 21st century...
http://windows.microsoft.com/en-us/w...load#sptabs=xp