Forum Moderators: open
Whenever there is a rendering issue, you really need to ensure that your code is valid first before you debug -- but you can't validate without validating TO some specific DTD. You could start with the partial dtd you have above, but this will keep your pages in "quirks mode", as you apparently developed them.
This much you can be sure of, Firefox is seeing something different -- perhaps in the mark-up before or after your include. You may simply be triggering an error recovery routine becuse of a mark-up problem.
I would definitely validate first:
W3C Validator - HTML [validator.w3.org]
W3C Validator - CSS [jigsaw.w3.org]
W3C List of Valid DTDs [w3.org]