Question text says it all. I have an '''iframe '''element in my HTML file where the '''src '''attribute at first specified a relative path ('''src="../../index.html"''' or''' src="../../"''') to another HTML document. Then an absolute URL ('''src="http://localhost/path/to/index.html"'''). '''Firebug '''is reporting a '''404''', and the reason is that it the HTTP request is to an index file in the '''parent directory''' of the currently loaded document. No matter what is set in the src attribute, the HTTP request is for an''''' index.html''''' in the directory that is immediate parent of directory of currently loaded document. This document works---iframe loaded and processed---as expected in''' latest IE version''' (other HTTP clients not tested yet). So it is any wonder why FF is doing this bizarre behavior. Things done: * cache was cleared * cookies were cleared * FF run with adds-on disabled * googled for some bizarre security issue: should not be cross-domain problem * MDN page for iframe checked for deviations from element coding standard: none found, document validates, including the iframe-loaded one * considered, but could not think of some about:config setting where iframe requests only are to parent directory * screaming and cursing at the browser