If your installation of Firefox suggests too many passwords when logging in to a site, I have a quick and easy solution for you in this post. This issue has bothered me for a long time now, and it was surprisingly hard to find any information on it. When I finally did find an answer, it was in an obscure, poorly titled Firefox forum post (https://support.mozilla.org/en-US/questions/1274937). For posterity, I decided I would post this solution here with my own screenshots.
The Problem
Firefox suggests way too many usernames and passwords when you’re logging into a site. If you look closely, it’s suggesting credentials for any site that is a sub-domain of the one you’re on:

These extra passwords were incredibly annoying to me. Not only is this an annoyance, but sometimes this long list of password suggestions can get in the way of something else on the screen.
The Solution
Update Firefox Signon Setting in “about:config”
Open a new tab. In the Firefox address bar, type:
about:config
You’ll receive a warning message that says, “Changing advanced configuration preferences can impact Firefox performance or security.”

When you see this warning message, click the button that says, “Accept the Risk and Continue”.
Next, you’ll see a search screen with another warning about changing these settings.

Edit signon.includeOtherSubdomainsInLookup
Type “signon” (without the quotes) in the search field. Look for the item named “signon.includeOtherSubdomainsInLookup“, and click the arrow icon next to it to toggle its setting from “true” to “false”.

As a result, the setting, signon.includeOtherSubdomainsInLookup, should now be set to “false”.

Passwords “From this website” Only Now
Finally, you can close the “about:config” tab and try logging in to one of the sites you noticed this problem on. You should only see suggested passwords from the website you are currently on. This is how suggested passwords have worked in the past, and it makes the most sense to me.

Let me know if this solution helped you or not. You can comment below this article or Contact me here.
Thanks for stopping by!
I got the exact same problem, and solved using your description. Thanks!
Great! Glad this helped!