Rated 4 out of 5 stars

Very useful indeed.
However there is no explanation to change the setting that is asked at first when we install this addon. This setting allows to use the key password across different web browser or not. Where is it?

This review is for a previous version of the add-on (1.1.5.1-signed.1-signed). 

I'm not sure I understand the question. You can set extensions.macos-keychain@fitzell.ca.startup-import-prompt to true to be prompted again to import - this just does a one-time import of passwords stored in Mozilla and copies them into the Keychain where (at least in theory) other browsers can use them. It turns out browsers have all kinds of quirks so it doesn't always work out that way, but I have pretty good success...