Personally I'd love to see Firefox use the same Keychain for passwords (same with Chrome). It would make it FAR easier to switch browsers. I wouldn't even mind shared cookies and bookmarks for that matter. Let the best engine win, but keep it frictionless to move in-between.

I wish there was (and I hope someone comes along and tells us!) During my switch to FF, I kept the Safari prefs dialog open and then copy and pasted the passwords when requested by a site in FireFox. It was a total drag, but in a few days I was all set in cross-device password management with Firefox.

Yes, Safari 15 offers Password export.

You can just use Firefox Sync and Lockwise on mobile to manage passwords etc instead of using Keychain I suspect. I'm not sure of the import functionality of that ecosystem but it wouldn't surprise me if someone figured it out already and posted the code.