Natalie Open normally your first account as you open then to open multiple account of same website follow the below procedure.
This is for Google Chrome,as you can see in the screenshot below click on the button highlighted and in the menu there is a option of "New incognito window" then click on it and visit the another accounts. You can open multiple accounts by this process no limit.
This is for firefox as you can see click on firefox then on private browsing and another separate window with firefox in purple background will open, now open another accounts of a single website.