Regex Search is a subscription-based add-on designed for Firefox, facilitating HTML searches using regular expressions (REGEX). This tool allows users to perform complex searches and highlights results based on specified patterns, making it a valuable asset for developers and power users who are familiar with basic REGEX syntax. The add-on provides a template output for results, enhancing the clarity of the search process.

One of the notable features of Regex Search is its ability to save search profiles, allowing users to quickly access frequently used queries. However, it is important to note that this add-on is restricted from functioning on several specific domains related to Firefox accounts and services. Overall, Regex Search serves as a powerful utility for those who need advanced search capabilities within HTML documents.