In LibreOffice 3.6, Tools menu / Options / Load/Save / HTML Compatibility / Export / Netscape Navigator. Netscape Navigator disappeared since 2008 (and the last version - Net. Nav. 9 - was based on Mozilla Firefox 2). In short, it looks a bit outdated to export for Netscape Navigator.
Thank you for reporting. The description has to be changed.
Submitted patch to change the name: https://gerrit.libreoffice.org/#/c/1051/
Rob Snelders committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/core/commit/?id=5eb9c58c31ac0ab1042ef1c2cc8b61e932dddf8d fdo#56726 - HTML Compatibility for obsolete "Netscape Navigator" The patch should be included in the daily builds available at http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: http://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.
patch is commited