Current Firefox releases include sub domains as a suggestion in the drop-down list.

You can set this pref to false on the about:config page to disable this feature.

• signon.includeOtherSubdomainsInLookup = false

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Does this procedure change the value for one site or does it change the value for all sites on my computer?

This is about not only showing suggestions for the current domain in the drop-down list that opens in a login form, but also showing suggestions for other domains. So this affect a login form on every website. You will always see login suggestions for the current domain, so this feature merely extends the number of items that are displayed what can sometimes be useful if the website has moved the login procedure to another sub domain. With the pref set to true you will always get a long list of choices what might be confusing or some can see this a a privacy feature that exposes other logins. If you have a lot of logins then it also can take longer to populate the list by scanning all the logins.

I'm trying to support a soft wear leak in Firefox as reported by my internet protection Provider. I'm getting nowhere, but this leak needs to be patched. It may be dangerous.