Brett Zamir has released XqUSEme, a Firefox extension based on berkeley DB XML that "can perform XQueries on the current webpage under view (including even many malformed pages whose Firefox DOM representation was able to clean up), under external websites which conform to XML, or to other files stored in the database. One unfortunate catch is that due to an apparent bug in Firefox with its LiveConnect/Java features, try-catch exceptions do not always work, so, one malformed XQuery, and you have to restart Firefox to try again... :("
