Accounts manager is a browser extension that lets you keep multiple accounts connected at the same time and switch among them.
Accounts manager watches for requests made to Roblox's login endpoint and asks you if you want to save the account you logged in; when you click yes (ok) the extension will create a copy of the .ROBLOSECURITY cookie of the account you saved applying the same security measures of the original cookie to keep the same level of security for the account as if you weren't using the extension.