XQuery is a W3C standard language which is meant to be for XML what SQL is for relational data--i.e., the ability to search, sort, extract, and remold data. It offers powerful and yet intuitive searching based on XPath, has SQL-like syntax for the query portion, and has scripting features such as function and variable definitions, XML-inclusion, etc.
XqUSEme (registration required as extension is still under review) is a proof-of-concept (so far tested on Windows-only) extension which allows one to perform XQueries on external URLs, the currently loaded webpage (even if originally from poorly formed HTML), and/or XML (including well-formed XHTML) documents stored locally.
The extension now includes and interfaces with the open-source version of Saxonica's Saxon B by default, while optionally allowing association with an installation of Berkeley DB XML, as a database system has the advantage of allowing local storage and indexing (though this extension currently only performs the XQueries). Other popular native XML databases might also be supported (e.g., eXist), especially now that the extension has minimal support for the open-standard (XQJ) for Java and eXist is in the midst of getting such an API. Berkeley DB XML was the initial choice by the extension developer for its support across many languages (C++, Java, Python, Perl, PHP, etc.) (besides its also being open source).
At present, the extension works simply by using LiveConnect to work with Berkeley DB XML's Java API (and via a Java wrapper class which circumvents LiveConnect's inability to handle some types of Java exceptions properly).
However, use of the approach of the Java Firefox extension might be used to turn the above extension concept into an XPCOM component (giving it access to all open windows), and possibly implementing it in C++ instead, which is the database's original language of development.