Rated 3 out of 5 stars

You said: "Passwords in the keychain are shared by all kinds of apps, including Safari".

That doesn't seem to be the case. I installed the add-on but then I don't see any of my old Firefox saved passwords appearing in Safari. To be fair, if I now add a new saved password in Firefox (after installing this add-on), it does show up in Safari.

This is a bit disappointing though. I was hoping my existing Firefox passwords (before I installed this add-on) would also be migrated to Safari.

This review is for a previous version of the add-on (1.1.1-signed.1-signed). 

It will do as you expect but import was broken

The first time you run firefox with the extension loaded, it will prompt you to ask if you want to import your existing passwords. Unfortunately, this was broken in 1.1. Version 1.1.1 fixes the problem, though you'll have to set the preference "extensions.macos-keychain@fitzell.ca.startup-import-prompt" back to true and restart firefox if you want it to prompt you again.