- I didn't know this is the expected behavior of FF
- I'm paranoid about getting locked out of certain accounts by stupid accidents (particularly on badly designed web sites). I temporarily copy old password too, when changing to a new one.
- I do wish Firefox's password manager maintained a history of previous passwords like LastPass does.
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.