To me the best part of Firefox was its unique ability for add-ons to customize the user interface and add significant new functionality. The architectural change in 2017 (version 57) was necessary but the failure to prioritize the re-implementation of the lost API functionality is dooming the browser.

Firefox's incredibly vibrant add-on ecosystem was a huge differentiator and competitive advantage over other browsers. I still use FF as my daily driver but I have to apply UX customization with CSS just to make it livable. Fortunately, there is an active community at www.reddit.com/r/firefoxCSS which eases the burden.

You mean APIs to reach into browser internals and escape sandboxing?