XML Viewer Plus is an open-source web extension for Firefox aimed at web developers and IT professionals who need to view and manipulate XML files efficiently. This tool incorporates support for CSS and XPath selectors, allowing users to perform complex queries for effective data extraction. It also features robust text search options, including Regular Expression capabilities, optimized for case-insensitive searches. With its support for both `querySelectorAll` and jQuery-style CSS selectors, users have flexibility in accessing and analyzing XML data.

Designed to handle large XML files over 1MB, XML Viewer Plus is particularly useful for developers dealing with substantial datasets. It facilitates a streamlined workflow by allowing users to pass selected XML code from a webpage via the context menu. However, it is important to consider that this WebExtension operates only on sites that allow content scripts, which may restrict its functionality on some webpages.