Currently, I use both Konqueror and Firefox as web browsers; however, they don't share bookmarks. It would be nice if they could use a common bookmarks database -- perhaps a good opportunity for a freedesktop standard or RFC; as a workaround, a tool that would synchronise their databases, collecting new and deleted items from both, would be useful.
Another approach would be to forget their builtin bookmarks support, and build a CGI system for managing bookmarks; this loses the convenience of "add here", though.