Are you able to save the login data on other sites?

Warning: There are some web pages that do not allow browsers to record or fill in the log-in information.

But if you go to the Mozilla Add-ons Web Page {web Link} (There’s a lot of good stuff here) you can try a Form Filling add-on. I found that the form-filler add-on I use is not affected.

Only this site not allowing save. :/ Not option save in Manager passwords.

boruts said

Not option save in Manager passwords.

What?

Type about:preferences#security<enter> in the address bar. Look under Logins,

Note: For Firefox v56+ its; about:preferences#privacy Look under Forms & Passwords +++++++++++++++++++++++++++++++ Turn on Remember Logins. To check the passwords you have listed, press Saved Logins.

Press the Exceptions button and see what's inside.

There is nothing in exclusions. One page is acceptable, apparently the page has a form not recognized by the password manager.

FredMcD said

. . . . the form-filler add-on I use is not affected.

You can paste this code in the Browser Console to add a login to the Password Manager in case Firefox doesn't detect the login form. You need to enable the command line in the Browser Console.

• https://developer.mozilla.org/en-US/Tools/Browser_Console

Firefox won't offer to fill the name and password via the Fill item in the context menu of the field, but you can open the Password Manager and use Copy in the right-click context menu for the name and password. You can edit name and password fields in the Password Manager with a double-click on these fields.

signon = '[{"username":"<name>","password":"<password>","httpRealm":null,"hostname":"https://mbok.pgnig.pl/"}]';
newSG=prompt("Logins JSON: [{},{}]",signon);
newSG=JSON.parse(newSG);
ChromeUtils.import("resource://gre/modules/LoginHelper.jsm");
LoginHelper.maybeImportLogins(newSG);