It happen because of your cookies,
if your cookie expired the username will be removed automatically.

Is this in a login form or in another input field?

If this is a login form, do you still see them all in the Password Manager?

You can check for the presence of multiple, uppercase or lowercase, versions of that name and password or entries with an empty name.

• Tools > Options > Security: Passwords: "Saved Passwords" > "Show Passwords"

I think that I described this issue wrongly. All usernames and passwords are saved correctly. All usernames also exists in drop-down list, but i need to scroll down to see the third saved username, because only two of them are displayed. Before last update, I could pick up third username immediately from drop-down list, now I have problem to quickly switch to another account. And that is what I need. I want to see all three usernames in drop-down list. So how I could force firefox to show at least three usernames in drop down list as before?

Works fine here. I can see at least four user names in the drop down list of a login form.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes