you should be able to add a "new tab" icon to your navigation toolbar. Right click your toolbar and there should be an option to "customize" it. Find the "new tab" icon and drag it to your toolbar, where it should stay from now on. Then to open a new tab all you ever need to do it left-click that icon.
If you mean keeping the previous tabs when you open a new session then yo will have right click either in the tabs bar itself or highlight 'Bookmarks'/'Bookmark All Tabs' = <CTRL> + <SHIFT> + <D> = BEFORE you close it down and choose 'bookmark all tabs'. This will put all the open pages at the bottom of the Booksmark list and fill it up PDQ.
If you just want a shortcut to the 'File'/'Open New Tab' then go into 'View'/'Toolbars'/Customise' and drag the 'New Tab' icon into your toolbar - I have mine between 'Stop' and 'Home Page'