This <div> is not editable.
If editing is enabled, this <div> will be editable...
You could choose to edit in HTML Mode by serving the document as text/html. The code produced is not valid HTML because the tags are formatted as XHTML tags. E.g. <br> becomes <br />, etc. With most modern browsers this doesn't cause much of a problem so you can get away without producing valid markup. When saving files you have a choice of saving tags in upper case (the default for HTML) or in lower case if you want to be XHTML compatible. Also note that the eDOM used by Mozile has been written for XHTML rather than HTML editing, although it could be changed to produce valid HTML.
Editing in HTML Mode has some advantages in that you can still edit a page even if it is not well formed. If you try editing an XHTML page served as type application/xhtml+xml and it contains even a single error in the markup all you will see is a parsing error message. If you are using lots of scripts server side to dynamically generate pages small errors could easily creep in unless you check the document is well formed XHTML before you serve it. (Even if you do check it, you still have to somehow correct it before serving it.) Unless you are using an XML serializer server side it seems unlikely with a large dynamic web site that you could guarantee all pages to be well formed.
More likely you want to take advantage of the XHTML editing features that Mozile has been designed for. To do this you need to serve up the page as application/xhtml+xml rather than as text/html. For XHTML editing to work your pages must be well formed. You can not expect to get away with writing sloppy XHTML code and expect Mozile to work.
This seems easy enough provided you run all your code through a validator such as Tidy, but you have to remember that XHTML is not HTML. If it was there would be no point in changing! In practice, this means you need to take more care over the HTTP headers, how you specify the character set, and whether you include the XML declarations at the start of your documents. Finally, you need to note that Internet Explorer and many old browsers will not handle the application/xhtml+xml Mime type. Hence the need to switch between different Mime types.
This test document shows the best of both worlds. How to serve up pages as text/html for viewing in any browser, and how to serve up pages for editing in HTML and XHTML Modes.