How can I use multiple tabs in Fully?

You can specify multiple Start URLs (type one URL per line and press Enter to start new line). The URLs will be opened on separate tabs. You can enable visible tab flaps and set colors in Toolbars and Appearance settings. In Web Browsing Settings you can also enable Swipe To Change Tabs to be able to change between tabs by swiping left or right.

There are also some JavaScript and REST APIs available to manage tabs programmatically. You can still use the usual JavaScript methods window.open() and window.close() to open and close new tabs.