Millions of people use web browsers daily. Extensions can enhance their basic functions. As the use and development of browser extensions grow, ensuring adequate code coverage is essential for delivering high-quality, reliable, and secure software. This paper introduces IBE.js, a framework to monitor and assess the coverage of browser extensions. IBE.js conducts an analysis of the main JavaScript files, background pages and content scripts, of 4,495 browser extensions from the Chrome Web Store. By utilizing a blank HTML file, we found that on average, more than 33% of the lines in these scripts are executed automatically. This coverage represents the number of lines executed by default, without any influence from user interaction or web content.Notably, IBE.js is a versatile framework that can be utilized across various platforms, ensuring compatibility with extensions from other web stores such as Firefox, Opera, and Microsoft. This enables comprehensive coverage analysis and monitoring of extensions beyond a single browser ecosystem.