I'm very thankful this is the current design. When I noticed it, I needed it to do exactly what it does, and was honestly quite pleasantly surprised it worked this way.
You see, I'd just tried to create an account on a website with a slow network link. The website then failed to load. I wasn't sure if my account had been created or not. I always wait for the account creation step to succeed before I save a password, so I hadn't yet saved this random password.
I was a bit worried I'd have to go through a lengthy password reset process, on my slow internet link. Fortunately for me, going back to the account creation page simply popped up the same password, so I just hit the "Sign Up" button again, without worry of losing my password again.
- 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.
EDIT 2022-12-20: There are at least 3 cases where this is desirable within a short period of time:
Filling password confirmation fields on the same page if we were not able to automatically do so.
Filling the same password on the next page
The password didn’t save on the change form so you need to fill it on the log in page.
Bug 1551723 will give the user the option to choose a new password.
You see, I'd just tried to create an account on a website with a slow network link. The website then failed to load. I wasn't sure if my account had been created or not. I always wait for the account creation step to succeed before I save a password, so I hadn't yet saved this random password.
I was a bit worried I'd have to go through a lengthy password reset process, on my slow internet link. Fortunately for me, going back to the account creation page simply popped up the same password, so I just hit the "Sign Up" button again, without worry of losing my password again.