That's odd. You'd think Firefox would have forgotten about that visit long ago. Here are some things to check:

(1) Do you use Firefox's Password Manager to save logins? Please check there to see whether you can find and remove her login:

"3-bar" menu button (or Tools menu) > Options > Security

Click the "Saved Passwords" button to review and remove login that you no longer want.

(2) If it's not there, do you have any Gmail-related add-ons, such as new mail notifiers or toolbars, or other password managers? One of the add-ons might have stored the login. You can check on the Add-ons page, either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then on the right side consider whether any might be relevant. Often an Options button would allow you to check the extension's settings and clear out any saved login.

(3) If it's not an add-on, you might at least try clearing all Google cookies on your Firefox. To do that, I suggest using the Permissions Manager page. Type or paste about:permissions in the address bar and press Enter.

The left column will start filling with a list of sites. In the box above the list, type or paste google and Firefox should show only Google-related sites. Click the first one, and then on the right side, clear the cookies for that site. Repeat for all Google sites.

Then try Gmail again.

Any improvement?

This frequently happens with Gmail accounts, when you try to log into multiple using the same Firefox profile, Google's cookies tend to get confused into which should be the default account.

First, go to accounts.google.com, and log out of all accounts.

Then, using the steps at Viewing and deleting passwords, remove any passwords for your niece's account.

finally, go to mail.google.com, and log into your account. You should be all set to go.