You can open it by typing about:home in the search/URL bar. If you right-click one of the existing icons on the same row and choose 'Customise Toolbar' you get a screen-load of icons you can drag out. One of these is Home which you can just click to display it — see screenshot below. Both the above presuppose that you've got the URL of your desired Home Page set-up. If you haven't click the three horizontal lines at the top right and choose 'Settings' then 'Home' in the column on the left. Next choose 'Custom URLs' in the 'Homepage and new windows' box. Enter the URL you want into the box that will come up below.