Why Firefox doesn't suggests passwords from Keychain Access?
Using Firefox Nightly there is no support for Keychain Access, passwords not saved/read in Keychain Access
Opaite Mbohovái (1)
There was this add-on :
https://addons.mozilla.org/en-US/firefox/addon/keychain-services-integration/
But that doesn't work anymore, as the developer didn't migrate his add-on to webextensions.
Note : Your passwords are stored in the key4.db and logins.json files. For more information, see : https://support.mozilla.org/en-US/kb/password-manager-remember-delete-change-and-import
Moambuepyre