Look into why the XHTML namespaces were needed in bug 1482256.

Taking a stab at a priority to move this out of the triage list. Adjust as necessary.

The main issue here is in browser.xhtml we create the script tags in the XUL namepsace which doesn't have special handling. In a XUL document the script is loaded by nsXULContentSink as soon as a script tag is encountered[1], but the element itself is just a plain nsXULElement. So when we load browser.xhtml, a plain nsXULElement is created for the script tag, but it is never loaded because the nsXULContentSink is not used. Whereas when we add a XHTML namespace to the script tag, there is special handling for loading the script when that element is closed[2]. [1] https://searchfox.org/mozilla-central/rev/5a18fb5aeeec99f1ca1c36a697082c221189a3b9/dom/xul/nsXULContentSink.cpp#779 [2] https://searchfox.org/mozilla-central/rev/5a18fb5aeeec99f1ca1c36a697082c221189a3b9/dom/xml/nsXMLContentSink.cpp#595