Re: Some Gaps
You can be a happy and productive Firefox user without ever running into the above. Even the developer tools inside every Firefox are likely seldom used by the average browser user.
The gaps are not about Firefox, the browser, but prevents Firefox/Servo/Spidermonkey/etc, i.e. Mozilla's technologies, from participating in the larger eco system built around browser technologies. It's behind-the-scenes, below-the-water-line stuff mostly and support all sort of developers. Without Firefox/Mozilla tech, the default (often only option) is Chrome. It's become entrenched, cfip. Electron which is the "Chrome run-time". Slack and VSCode are both Electron. It further cements Chrome's rendering engine and the V8 JavaScript engine as _standards_.
Having Firefox be a first class platform for developers is really just about JavaScript in the browser, It's about the development tools connecting to the browser's debug and tracing APIs, code hot reload code and more. Chrome has that solidly sown up and one cannot substitute Firefox for that. It means the developers use Chrome and take it as the gospel standard.
The Firefox browser should be a flagship/demo implementation of Mozilla web technologies, not the one and only use case.