If an image's src is set to a full URL, but is not from an external site, it gets rewritten incorrectly in Firefox 3.6 (actually any browser for which $.support.dynamicBaseTag is false, but that only seems to be Firefox < 4) if it's from a higher-level folder and the page is loaded via AJAX. I've built a quick example based off the list test page (http://jquerymobile.com/test/docs/lists/lists-icons.html):
Broken (AJAX loaded): http://next.gen.nz/bugs/jqm-images/#/bugs/jqm-images/icons/index.html
Working (standard HTTP load): http://next.gen.nz/bugs/jqm-images/icons/index.html

This example uses the JQuery Mobile code directly from the http://jquerymobile.com/test/ so it should be able to be used as a final test if/when a fix makes it that far.

The problem is the following replacement, on line 618 of jquery.mobile.navigation.js:
//if full path exists and is same, chop it - helps IE out
thisUrl = thisUrl.replace( location.protocol + "//" + location.host + location.pathname, "" );

The comment suggests that it is for IE's benefit, but IE doesn't run this code as $.support.dynamicBaseTag is true in IE (7+; IE6 doesn't appear to be supported by JQuery Mobile at all). Removing this line does not appear to have any negative consequences.