Having to take such steps sounds like what you're using isn't working right. Fortunately Firefox doesn't need you to do this.

Clipboard history can also be helpful in this context (I use Unclutter on macOS, YMMV), tho it has its pros and cons.

But I have a use case where I really do not want this: when I sign up for multiple accounts in the same browser session, I expect a different password when I've logged out. I use Bitwarden and a text file in this use-case though; not Firefox. Because Bitwarden allows password sentence and I'm supposed to hand over the account to someone else.

I also do this, and I double check it in Safari before closing any windows. It's become a good habit, especially with things like Disney Plus but logged in via Hulu credentials. Example, for some reason sometimes www.hulu.com works but auth.hulu.com doesn't know that I changed my password, so I can easily paste the new one into the appropriate field and everything works.

More or less it's what I do except that I generate the password with a password manager (one of the Keepass family) and store it there.

Rationale:

1. I'm not tied to a browser, even if it is Firefox.

2. I can use the password with any other browser on the same or any other device.