Firefox 1.5.0.5 has a bug that causes it to be unable to play some streaming media, including Windows Media and Real Media. The MozillaZine forums and Mozilla Bugzilla suggest that a 1.5.0.6 release to fix the bug may be coming soon.
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https://bugzilla.mozilla.org/show_bug.cgi?query_format=specific&order=relevance+desc&bug_status=__open__&id=346167