Also dev tools are so much better in Fx and Chrome...
Besides Safari's lack of extensions I find it to be more or less on par with Firefox, assuming you're in the Apple ecosystem. The dev tools feel extremely polished and feature complete. The only thing Safari's dev tools lack IMO is no fancy CSS support for things like grid or animations.
I occasional open up Firefox Dev Edition when I get a weird bug on a webpage and that sort of thing but besides that Safari has served all my needs perfectly.
Much less than that of Fx or Chrome. No support of extensions means I can't use something like https://github.com/vuejs/vue-devtools, no command menu, poor debugger, no performance\memory usage information etc. The list can go on and on. Safari's dev tools are about at the same level as they were in Chrome or Fx 5+ years ago.
As for the extensions part... well, not everybody needs them, I guess, but I can't really imagine my browser without at least, 1P, uBO, Stylys, Tempermonkey, TreeStyleTabs, Tab Session manager, Bloody Vikings! and All Mangas Reader.
Safari's inspector always feels obtuse compared to Chrome or Firefox but maybe I'm just not used to it.
I use Chrome for development. It has lots of helpful extensions, like the React and Redux devtools, extensions for cookie editing, screen measuring, JSON formatting and so on. I often need to forcibly restart tabs (or the entire browser) due to misbehaving devtools, so it's great having a dedicated browser for it. Same goes for cookies and local storage state. It's completely separate.
I use Safari as a browser. I has the basic extensions (adblocker, etc.) that I need personally. Its development tools are terrible, but that's okay — I only use them in the very rare case where a rendering/JS problem only shows in Safari.