- I can't disable CORS in Firefox (yes, sometimes you have to disable CORS rather than modify the Allow-Origin header response, for example if you need to test against a production backend) (and, no, CORS Everywhere is not a sufficient solution).
- I can't inspect WebSocket frames in anything except Chrome.
- Safari does not allow self-signed certs over WSS (and there's no way to override it).
- Safari does not respect System-wide APC Config for Proxies.
There's a handful of other issues. Both Safari and Firefox do do things well, and often better than Chrome. For example, Firefox tends to actually handle standards correctly, whereas Chrome tries to be overly forgiving. And Safari's Develop and Debug menus are easily the best and quickest way to disable CORS or JavaScript, and examine service workers.
Unfortunately, some of the above issues are blockers.
I can test with Firefox on staging or in production, but not being able to test up front during development really impacts compatibility testing.
If another browser was as good or better for development, I'd be happy to use it.