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A general description is here.
Rgui has a new menu item 'Html search page' on the Help menu, which goes directly to the HTML search form. This is needed by users of Firefox 3, which generates incorrect links in search results when the page is accessed via the 'Html help' menu item (or via help.start()).
help.search() has to new argument 'searchEngine' to go directly to the search form: this is needed by users of Firefox 3 to workaround a bug in interpreting the search results pages.