While Safari doesn’t support importmap, it’s possible (and not too hard) to use them as a progressive enhancement so that it’ll still work, and safari users will get the benefit with 16.4. I did a writeup: https://qubyte.codes/blog/progressively-enhanced-caching-of-...
Firefox only started supporting import maps in v108 which was released 3 months ago. Previously you would need to use an experimental flag to enable it.