Added a conditional test at line 320 as this code breaks ajax modal wind...

There is an issue with ajax modal windows in Firefox as described in the bug report listed below.   We ran into this problem at NBC and the work around listed in 3152 does work.  I simplified the work around listed in 3152 by simply wrapping the code in classes/dom/Serialize.js with a conditional that checks to see if the browser is Opera, instead of replacing the whole block of code as suggested in 3152.

See bug report www.tinymce.com/develop/bugtracker_view.php?id=3152 for details on this issue