check discontinous selections in Firefox3

[alfonsoml]

According to  http://mozillalinks.org/wp/2007/04/discontinuous-selections-available-in-firefox-3/ , Firefox 3 has the ability in the selection to contain multiple elements, so we should review that everything keeps working because people might want to use it.

[martinkou]

I think it would be a good idea to add support for multi-range selections in CKEditor 3. The window.getSelection().getRangeAt(n) function on non-IE browsers has syntactically supported discontinuous selections for a long time already, it's just that most of them aren't allowing users to actually support multiple text ranges yet.