They can't out engineer Google on dev tooling, or the dev ecosystem. They just don't have the resources.
Chrome dev tools is vast and very rapidly evolving. Google has large evangelists teams dedicated to explaining dev tools or web tech in general. They are everywhere, on Twitter, Youtube, blogs, conferences, in web standards proposals.
It's Google all the way down. If I'm not mistaken, Mozilla doesn't even have any evangelists anymore.
Yet I still use Mozilla MDN far more than all of those google resources put together. They're not dead yet.
They need to go all-in on developer tooling.
The manifest v3 issue may help by poaching technical users from chrome.
They also need to stop being so "principled" about features such as WebMidi. If I have to keep another browser around certain websites that Firefox is too principled to support then why bother using Firefox at all.