XHTML is really quite different for a browser than HTML and currently CKEditor won't work that way. In order to make it work, you would need to find every piece of code that it isn't compatible with XHTML and provide an alternate solution, keeping in mind that there are also some differences between the XHTML modes of each browser.
Quick solution: keep using HTML as you are doing for IE.
Thank you. Sometimes the simplest solution is the best one. I can't remember why I was using XHTML, but as having CKEditor working on my site is quite important, I'm going to stick to HTML.
Re: CKEditor Not Appearing in Firefox/XHTML-Transitional
In order to make it work, you would need to find every piece of code that it isn't compatible with XHTML and provide an alternate solution, keeping in mind that there are also some differences between the XHTML modes of each browser.
Quick solution: keep using HTML as you are doing for IE.
Re: CKEditor Not Appearing in Firefox/XHTML-Transitional
Thank you for your prompt response.