The best alternatives to Firefox UI Fix are Mozilla, CSS Scan, and Hoverify. If these 3 options don't work for you, we've listed a few more alternatives below.
At Mozilla, we’re a global community of technologists, thinkers and builders working together to keep the Internet alive and accessible, so people worldwide can be informed contributors and creators of the Web. We believe this act of human collaboration across an open platform is essential to individual growth and our collective future.
Hoverify makes inspecting, editing, copying HTML and CSS super easy by just hovering over the element. Pick colors from anywhere on the page and take screenshots.
Collect visual feedback from your team and clients directly on your site. Toybox is the fastest way to create, collaborate and manage ideas on any website.
This tool measures the 'cumulative layout shifts' that occur when a page is loaded in the browser using data from the Chrome Performance API. These are then listed showing the 'layout shift' score and visualised on the webpage.